I'll give you my take for what it's worth...
 
1. What are some popular uses for it? What is most common?
 
Everyone will probably give you their take on it. For me personally, the most common is when instruments continue to lash out at me where the frequency is more the problem. I also use it for mastering to keep the little peaks and valleys in check after eq-ing. I prefer the multi-band over my treasured API 2500 due to having the right one. I can't speak for the Sonar multi as I haven't worked with it enough, but the UAD Precision Multi is the best I have ever used, hands down. It obliterates Waves and anything else I have really worked with. That said, I have also had really good results with the old Multi (not the Sonitus) that comes with Sonar. I'd imagine this new one is probably a lot better. I've messed with it, but haven't hard cored it yet.
 
2. What kinds of creative uses are people using it for?
 
I don't know about anything creative, but I guess you could come up with "anything a compressor can do, this thing can do while giving you control over frequencies". So if you use compression creatively, you can do that and control the bands.
 
3. What kind of problems can it solve that make it the best choice over other tools?
 
For me, the biggest issues where the multi is a necessity, is in low tuned bass guitars and high gain guitars that may be tuned super low, or they just have a bit more low end in them than they need.
 
For example, some 5 and 6 string basses will send out some really low notes. Let's set up a scenario. Say we have a 5 string bass dialed in and it sounds great until the guy hits that low B. The low B appears to jump out at us. Here's how we can fix it:
 
We can automate volume levels to fix it in that spot and EVERY spot it turns up

We can lower the level on the frequency by automating the eq EVERY spot this turns up
 
Or, we can use a multi-band and let IT take care of everything for us.
 
The low end on a low B comes in at about 30.87 Hz...so I say 31 Hz. So of course we are going to high pass first to control any crazy low end, but we have to check out how much pull from the player (each person has a different attack/execution of the instrument whether it be with fingers or a pick) pushes any frequencies out that make us hear them more. It may wind up being a note played on that B that isn't a B. So when we scan the file, we may have to control some other frequency. Whatever the case, once you know what frequency it is, you can set your multi to put a little compression on the offending frequency. So it will police that freq and will not allow it to cross the line of the threshold you select.
 
Another situation is high gain guitars that whoomf. Most times, the whoomf frequency on these types of guitars is 120-130 Hz. Slapping that multi on and having it police those freq's (along with some high passing) clears this right up. And it will only grab when the freq exceeds the threshold you assign. So you have loads of control over what you allow to sneak on through. :)
 
4. This may be redundant but, what can it do that nothing else can? Or is it just a case of being more convenient?
 
All of the above that I mentioned. Nothing works on literally compressing and controlling an offending frequency like a multi-band. The problem is, like compression and limiting, people over use them to the extreme and they can ruin a mix with pumping and breathing. Don't use it like an EQ...use it to control eq that cannot be controlled by literally adjusting the eq. For example, if you have a great sound...and only 1 or 2 parts get a little weird based on note changes....why mess with the entire eq curve for those two little parts?
 
The little parts could be due to the bassist pulling a little harder or his strings being closer to the pups in that passage...or there being a little more meat on the frets to make the sound come in differently....the multi can control all that without having to revamp your entire eq curve.
 
5. What situations would you reach for this rather than full band compression or a parametric EQ?
 
That's the thing, I wouldn't substitute it for an EQ and this (in my opinion) is why the effect fails for people. I use it in mastering to sometimes control frequencies that sound good that may start to press in other parts of the song. The same situation I mentioned in my answer to 4. Sometimes everything sounds great until there's a key change or something in a song, and then something sounds wrong. The multi can jump right in and keep you consistent by either allowing more freq's to be present, or by policing them to be less present.
 
To me it's more of a surgical tool when I don't get the right sounds from eqing and compressing individually. I like the multi for mastering because it just lightly polices frequencies from crossing the line. Sometimes I like a little more low end happening, but then here comes that part of the song where the bass does something different and it's too much....so I will lightly control that frequency with the multi, and call it day. Hope this helps. :)
 
-Danny
 

Sylvan
 
2. What kinds of creative uses are people using it for?

There's a particular situation with synths that I've found this tool useful for.  Let's say we have a sound with velocity mapped to loudness - a very typical mapping.  At quiet playback, the patch may have a nice blend of lower and higher frequencies.  But as you play louder, the higher frequencies make the sound too strident or aggressive.  You want more body when the loudness increases but not more brightness.  A two or three band compressor is a fine solution to this challenge where you leave the lower range alone and compress only the higher end.

I think they are great and the can solve a lot of issues sometimes.  I like them in mastering.  You can get away with a full range compressor on most things on tracks due to the nature of the sound being less complex.  On full mixes multi band compressors can be better.
 
A couple of things worth doing with them though.  It is good to solo the bands and hear what areas you are effecting.  I find it helps.  Also be prepared to alter the crossover frequencies.  Don't just use stock settings because I have found altering them can give quite different results.  Solo the areas while you are setting crossover frequencies too.  Smoother more transparent operation results when tweaking the crossovers and bandwidths etc.
 
The multiband can tame just a certain area of your mix well.  It can also when confronted with a real nice mix just dance over all the bands and then it is nice to get all the bands doing equal gain reduction too.  The sound or EQ won't change.  You can still get into light conditioning over a well mixed master.  Small amounts of GR eg 1-2 dB, slow attacks and low thresholds like 1.5:1 in most of the bands. Release settings are important too.  It can impart a pro sound to your master.
 
It's cool to be able to alter the make-up gain of each band as you can turn it into a sort of nice active EQ as well.  I find I can use less mix EQ  (pre compressor usually for me)  using the make-up gains in the bands instead to fine tune a mix EQ.
 
It is nice to be able to alter the ratio and GR of any given band.  And then all bands locked together at times when making adjustments but also you have to fine tune bands individually too for best effect.
 
I like the Sonar Multiband a lot as I feel it shows you a lot of information in one look.  The Studio One multiband is a bit complex and it frightened me off at first but after a while I got all over it and it is excellent as well.  Mixbus has one of the best though.  What that can do is limit the amount of gain reduction ie it can be set so it never drops below a certain level which is actually great. Eg -2.5dB etc  Even on a loud segment, instead of a band dropping way down by crossing over a threshold with a large amplitude, it still does some GR but then stops at some point.  Overall mix EQ can be maintained well that way.  Some of the dynamic range is left in any one band. That is a cool feature, some multibands can’t do that.
 
 
 

Weird timing on this, but MeldaProduction just shot out a "Multiband audio processing and crossovers" video just two hours ago (https://youtu.be/wHdRyb-2vzo almost 20min in duration). The video is basic in some respects, but definitely shows the capabilities of the processing. Excellent plug for their stuff though (they have MMulitBand "everythings," it seems).
 
 

Wow,
 
Those are some great responses. Thank you Danny, Jeff, dmbaer, mettelus, sven, b rock, and everyone else.
 
Danny, your detailed response really helped me understand. I think I get it now. Other responses helped clarify and confirm.
 
I used the new MB to tame the low frequency swell that happened on palm-muted parts in heavy distorted guitar. It would swell up on the palm-mutes but nowhere else. I put the MB on there and set it to reduce some low frequency by 3 or 4 dB, but only during those times. It really evened things out and work well.
 
Jeff, on this new MB that SONAR recently offered, it also has a "range" as you describe the one in MixBus having. You can set a limit to how much compression will take place. These two new plugs are really high quality and well designed.

I've been experimenting with parallel compression using the MB on mixes. Duplicate a mix and smash the 2nd with an MB comp, then blend in to suit. Helps to make mixes a little more reasonable on different playback systems and at different volumes. MB is also good for taming high end on a clangy acoustic guitar.