Again...a lot depends on your application. 
 
VocalSync: I record a lot of harmony parts and I like making them tight.
Drum Replacer: In addition to its obvious use, the Drum Replacer opens a lot of creative possibilities.
Theme Editor: Being able to customize particular IU elements for my particular workflow has been great, although I also use it to create themes that project well during workshops/seminars.
Boutique Suite: This is huge for me. Just about every project has at least one VX-64, TS-64 Transient Shaper, and/or PX-64, but the BiFilter 2 and Breverb also get significant use. As just one example, most transient shapers seem optimized for drums; I can use the TS-64 with guitar, bass, marimba, etc. to help make samples sound more "live." You can always put together processors to do what the VX-64 does...maybe. The way you can shape the Delay frequency response with it is genius, and perfect for vocals. I don't know of any other "one-stop" plug-in for vocals that's as effective, and getting a sound is easier than manipulating three or four separate plug-ins to try and do the same thing. All the other VX-64 modules (Saturation, EQ, etc.) are optimized for vocals as well. 
ProChannel: I use the Console Emulator, Tube Distortion, and S-Type Bus Compressor a lot. The Console Emulator is of particular interest because it models an audio input transformer as well as channel non-linearities. Back in the day I saturated input transformers with bass and kick all the time, and now I have the same effect in the virtual world by treating the Console Emulator NOT as a Console Emulator, but driving it super-hard to "saturate" the "input transformer." The Tube Distortion has two different distortion modes that are crucial when adding hard clipping to bass and particularly, drums which can bring up the average level without any of the artifacts of dynamics processors by shaving off the very highest peaks. I rarely use the Tape Emulator but every now and then, it's just what I need for acoustic guitar rhythm parts.
 
I don't find the QuadCurve EQ flyout in Platinum as important a feature as many people, because I need more flexibility with spectrum analysis. But it's good for "quick and dirty" response checks.
 
I think there's a tendency to downplay the included plug-ins because they're bundled and have funky graphics, but what makes them worthwhile is they offer unique functions. Then again, I use SONAR almost every day for professional needs. If music is just going to be a hobby, you need to decide whether your hobby encompasses the same kind of needs. You could buy third-party plug-ins to do most of what the above plug-ins do, but that will probably end up costing you more than lifetime updates, depending on which of them you need.

Ah yes, I forgot about BiFilter 2. That is quite good and useful, also for saturation/distortion fx.

I fully recommend Sonar Platinum. Platinum has everything CW has to offer in their DAW.  The other versions are cut down versions. As to the bundled plugins that come with Platinum they stand up well against any others. Thinking that they are not high quality is a mistaken belief.
 
The reason to get others is if they fill a need that is not being filled by the ones already there. I just bought the Arturia upgrade to V Collection. The reason, Sonar Platinum doesn't have those synths. Nor does anyone else.  

I was using Pro until about a month ago. The main reason I upgraded was that I was nearing a Pro renewal. While I didn't have to take it I figured a renewal and some extra bucks and I wouldn't have to worry about it and I would get Plat in the process. The extra instruments was another plus.
 
As to the ProChannel stuff, I don't really use them enough to singly justify the extra cost but they may come in handy sometime. Maybe I just don't have a good enough ear to determine the subtle nuances of one reverb or limiter over another...

DrLumen
As to the ProChannel stuff, I don't really use them enough to singly justify the extra cost but they may come in handy sometime. Maybe I just don't have a good enough ear to determine the subtle nuances of one reverb or limiter over another...

 
Don't underestimate yourself, there are many more similarities than differences. You will find special-purpose variations like the FabFilter Pro Q that take processors to an extreme in terms of functionality, but given that it costs $179, it doesn't give me $179 more of what I need compared to SONAR's LP EQ. As to reverbs, they all sound different - which doesn't necessarily mean "better." I still use the Sonitus reverb sometimes; between that, REmatrix Solo (convolution), and Breverb (algorithmic), and Softube's TSAR 1, I'm covered. 
 
The only processor where I've found nothing equally good, in SONAR or from any other 3rd party manufacturer, is the Waves L3 Multimaximizer. I also think the Exciter in Ozone is stellar, but I use both of those only when mastering.
 

Its odd though. I don't believe anyone listening can say oh yah thats a Fabfilter EQ on that piano. Or that sounds like Waves on that bass. If it sounds bad then its an inferior plugin. If not I don't see an issue. Only the person mixing has any idea what equipment was used.
 
Really, if one can tell what plugins as processors were used I submit something is wrong with that mix.  

John
Its odd though. I don't believe anyone listening can say oh yah thats a Fabfilter EQ on that piano. Or that sounds like Waves on that bass. If it sounds bad then its an inferior plugin. If not I don't see an issue. Only the person mixing has any idea what equipment was used.
 
Really, if one can tell what plugins as processors were used I submit something is wrong with that mix.  

Well the sound and the functionality are two different things, because the functionality may be able to let you get a unique sound. No one could tell I was using the Waves L3, but they could say "wow, that mix sounds really good" because I can adjust the amount of compression for individual bands easily, and bring a little lift where needed.
 
I A-Bed the L3 against several different products, and to my ears there was a significant difference that was better than anything else I tried.
 
Closer to home, the different curves on the QuadCurve can make a huge difference. If you know how to use the functionality, you'll get a better sound...but that doesn't mean anyone will know how you got that sound.

"but that doesn't mean anyone will know how you got that sound."
 
This is the only part I'm talking about. 

Anderton
 
Closer to home, the different curves on the QuadCurve can make a huge difference. If you know how to use the functionality, you'll get a better sound...but that doesn't mean anyone will know how you got that sound.

@Anderton,
 
I think you're saying that there are two useful but independent attributes:
1) The sound quality of the processor
2) The functionality and user interface that make it easy to use the processor effectively
 
I think you're implying that the QuadCurve's sound quality is up to industry standards, but the functionality and user interface are what makes the QuadCurve exceptional.
 
Is that what you mean?
 
[Edited]

I will also argue that the interface of a plugin makes a difference in the end result. While this should in a technical A/B situation not matter, in real life if the interface is intuitive and fast I'll dial in the right settings quicker/easier and will stay "in the flow" more than if I have to fight with every plugin (or even have a lot of extra clicks/drags). One reason I love ProQ(2) is because it goes right to the core of what I want to do when I first open the plugin. I can do most things with a single click/drag anywhere on the spectrum. I definitely can't tell the difference in sound between ProQ and the QC (assuming identical curves and no linear phase) but it just suits my workflow.