I suppose the question is still valid if they're on your machine, but never get used... or VERY rarely.
 
At one time I had waaaaay too many plugs... a lot of that free junk and realized I never really used them. They're now off my main hard drives.   I'm not using X3 as of yet, but the idea of JUST X3 (and maybe a stereo editor) sounds appealing.
The ProChannel, Breverb, etc seems to fill that need and I don't need a pile of "boutique" or different reverbs, flangers, etc.  I like going to my FX list and not scrolling for days.  For ME, if I can't get a decent sound out of these tools, it's not the software...  it's garbage in garbage out.  But that's me speaking about me.
 
I know some swear by their UAD or Waves or Valhalla room ....or just have a BIG pallet to chose from.  .. but the idea of a native, stripped down setup seems really appealing at this point.
 
Excluding VSTi of course  :-)

I'm almost exclusive, I only have the TSAR-1R reverb plug-in that I use sparingly, but since X3's BREVERB 2 came out I don't need the TSAR-1R anymore. Other than that I don't have any more plugins out side of Cakewalk's Pro Channel plugins....just don't need them. And I'm getting great sounding test mixes.

looking for the "like" button...

I couldn't do without Valhalla Room - I just honestly prefer the sound of it over Breverb. However since X3 I've found myself using the Quadcurve EQ a lot more and 3rd party EQ's less. However I'll mainly use it for filtering and cuts, while still using my beloved Waves V-EQ for boosts. 
 
I'm also using the PC4K S-type compressor on tracks a lot more. Like if I'm putting a guitar through an amp sim, I'll give it a little gentle compression with that module before it hits the sim. But after the amp, I'm still using Renaissance Axx a lot, just because I seem to be able to get what I'm looking for very quickly. 
 
So I guess I'll never use stock plugs exclusively, but I am using stock plugs a lot more than I used to. And if all I had were the stock plugs, I don't think it would bother me much. Some of them are superb. 

I find the stock plug ins in sonar to be a bit kids play sounding when compared to UAD and Waves plugins so never use any of them other than the EQ if I want to make a quick frequency cut

I forgot, I also have Guitar Rig 5, I like it, but I rarely use it. I love the guitar tuner that comes with it. That is so handy.

djwayne
I forgot, I also have Guitar Rig 5, I like it, but I rarely use it. I love the guitar tuner that comes with it. That is so handy.

Tuners drive me nuts! I can never get past the fact that when you strike a note, the pitch meter fluctuates. I never know whether to go by the initial attack pitch, or the pitch it settles on (and even that that fluctuates sometimes). I've tried to use them but always seem to go back to my trusty old 440 tuning fork. Once I've got that A string down I'm all set. 

mixmkr
Who's using Sonar X3 exclusively - no outside plugs

 
Me

I guess it is a matter of taste, but in principle X3 producer is to PRODUCE professional music as it is. Some of us also add other plugs to minimize hardware or facilitate tweaking after recording (e.g.,  Guitar Rig 5, Easy Drummer, Amplitube, etc.). 
X3 is great with the addition of many new features (e.g.,  Melodyne, Addictive Drums over Session Drummer, Nomad filters, etc.). TH2 is one of the best plugs for guitar and in this case I think it is worth it to upgrade to the full version for another hundred bucks. 
Enjoy producing music! 

Almost there.  Sonar's lack of full featured sampler/slicer means I have to use third party for that.  I do find myself using built-in plugs more and more. Sometimes i think if one did a Pepsi challenge sort of test with people using plugins, the results would be a lot closer than conventional thinking would suggest.  I'm not saying that each and every Sonar plugin is equal too or better than third party plugs, but that for many the price difference does not offer the equivalent difference in performance.