So, why is MONO important? NOT!
 
I often drive through east lansing, and the MSU campus.
 
I have been way happy to see dozens of kids wearing headphones.
 
Staples is selling such headphones for more than $300.
 
So, of course mixing with/for headphones is a requirement.
 
But why worry about MONO.
 
Nina Hagan used way-happy polarity adjustments to broaden the sound stage, in the '70's.
 
Sure these tracks in MONO would be not the same.
 
But why should I care now?
 

You don't need to, but mono compatibility is still preferred for any AV work esp., since you never know what format the audio will be tortured into.  Some web stuff is still gets mono music, although TV is mostly broadcast in stereo these days.  It used to be much more important - not for home recordists but commercially.
 
It is also an excellent method of checking mixes.  If you can hear the separation in mono, you know it will sound good in stereo.

Mono is a great tool to have under control.  Your music ends up in stereo for sure but mono work earlier leads to better final stereo images.  I prefer the mono speaker to be a single speaker in this case and separate to the main speakers. Mono sounds better to me being heard from one speaker alone.
 
It can be used in a technical way to check mono compatibility with say a hardware synth sound that has used excessive phase devices in its final effects processing.  I once had a hardware synth patch that sounded amazing in stereo and collapsed to a heap in mono.  Loss of level and terrible tone change.  In the end I shifted the phase of one channel by 90 degrees I think and narrowed down the image.  It still sounded stellar wide in stereo but collapsed very nicely in mono not loosing level now and sounding robust.  It means it will always sound wide but if for any reason the stereo image is collapsed to mono the synth part will stay behind and remain.  Good for checking drum buses too for solid mono sound but just nice wide stereo sound.
 
One artistic creative device might be jumping from a full wide stereo image to a total mono image.  For a short time.  eg 4 bars.  Getting the mix to sound good in mono takes some practice too.  It forces you to make more changes to similar sounding parts to separate themselves.  Back in stereo once panned etc they sound clearer then.  Turn them down now a little and they are still heard.  Maximum illusion minimum voltage.
 
Another creative effect might be jumping from the normal stereo mix to a mono mix again but panned to one side with the out of phase copy on the right side.  ie in phase L and out of phase R.  When you use this effect you might want to temporarily edit the EQ of both sides to make it sound better for the duration.
 
Listening to your mix summed in mono on a small speaker at low volume tells you a lot too. If there are too many things going on it will be revealed in this check.  All the parts are lined up with each other in the same spot.  It clutters everything.  After pealing back some parts and simplifying tracks everything will sound clearer in mono.  Just the right amount of things going on.  Back in stereo and panned it will sound twice as good.
 
It is also good to listen to just the left channel only to make sure the vocals (or lead instruments) and important stuff is still present in the mix.  And same for right channel.  Listen to what is just there.  Many times I have been in cafes or restaurants and there are two speakers set up but only one is working.  I have composed music for AM radio and in many TV soundtrack situations where my music ended up being summed L+R for whatever reason.  I got caught out badly once in that situation.  (A very important part nearly disappeared because I was being smart!)  It is good to know how that is going to sound by doing a mono sum check.

Just name your album "For Headphones Only" and you'll be OK.
 
The process would be so easy if we could be assured that everybody'd be hearing our stuff only through quality headphones. (Unfortunately, those cans you see the kids walking around with are not high-quality, just overpriced.)
 
But the reality is that if the music is successful, it'll end up being played over a nightclub's (mono) sound system, or used in a TV soundtrack (might as well be mono), or played on radio (ditto).
 

Yeah, it is a good way to make sure your music is mixed properly.  There are only two tracks in stereo, and there is a lot of information shared between them most of the time.  Summing to mono is still the best way to expose and amplify problems not only in phase, but also in balance.  
 
As an end medium for serious listening, there is no mono anymore, not even in cell phones (okay maybe cheap ones).  Live PA and strip malls still play music in mono, but with audio quality that bad,  mono compatibility problems are the least of your worries.  

Michigander I am! You're far out there though.
 
Mono is pretty important for me... I've noticed that when things are in mono you may get disgusting frequency buildups, masking and overlapping. Someone may be listening on a phone or a single speaker. I'd like to mix to account for every possibility.
 
what bitflipper said also is true (i just didn't wanna repeat info:D). ^^

A few more reasons why mono matters.

iPads and other tablets may claim their built-in speakers are stereo but in practice they're often on the same side of the device so are effectively mono in that any phase cancellation effect between left and right will be massive. Laptop speakers, even if a few inches apart,have the same problem.

You have no control over the listening environment. Rather than follow best practice, people often put hifi and surround speakers in all kinds of strange places. As in putting the left speaker under the sofa and the right speaker across the room facing it up near the ceiling on a wall bracket so they're less visually obtrusive. Or side by side with one angled left and the other right and the listener in the dead zone between them.

You have no control over how end users or intermediate stages between you and them might lower your stereo track's resolution. Or how someone might start with downloading high resolution mp3 then compress that into a much smaller mp3. When converted to low bitrate mp3 stereo phasing issues can reveal themselves through headphones as the entire track switching from side to side like a drunk staggering down the road.

Finally, mix the low end with stereo separation and it can sound really weird through headphones as all the weight of the track switches sides or sticks on one side. Again, not pleasant to listen to.

OK
 
I have used panipulator for years to check stuff in MONO and channel flip.
 
I sorta thought the headphones seem to be an improvement over ear buds. And in the past year I have noticed more wearing the phones. But I defer to the BIT man about quality.
 
So: what is the point of stereo?
Only for our temporary fun cuz our happiest fun may reduce usefulness?
 
Fun listening to that old Nina stuff.
 
 
 
 

YouDontHasToCallMeJohnson
So: what is the point of stereo?
 

Stereo means the hi-fi shop gets to sell everyone two speakers instead of one?
 
More seriously, you've made a very valid point there. Most live and club PA systems are mono and lots of great music has been recorded in mono and sometimes still is. For a bass/drums/one guitar band the best mix can easily be just to send everything straight down the middle like they did in the 1950s and early 60s.

I'm starting to toggle the mix bus to/from mono from time to time, just to listen to the mix.  From what I gather, if I can hear each track in mono and still have a decent balance in the mix, I'm doing something right:)