Great stuff here everyone. I've been debating using them, and I just wanted to run it by the forum to see how you guys feel.  So far, it looks like a good thing. I don't use the interleave button much anyway (just to mono-ize) the kick and bass.
 
 

I use Kontakt a bunch, and due to its multi-timbral capabilities (I can load up multiple separate instruments into a single Kontakt instance), I just always use separate audio and midi tracks.
 
Track folders help me minimize the extra real estate chewed up by the additional tracks, as well, so it isn't bad.
 
Lastly, for some keyboard sounds, usually an organ, I will often have a separate left and right audio track, and will put a slight delay on one side of the signal, which acts as a really nice widener.  The technique is credited to a German guy from I think the 1930's, but I can never remember his name - I always end up calling him Hasenpfeffer (German for rabbit stew).
 
Bob Bone
 

Maybe it's not so much that I don't like Simple Instrument tracks, but that I really like what you can do with synth track folders - I detailed those reasons in a tip of the week. The ability to show/hide groups of MIDI and audio tracks with a single click is fantastic when you have synths with multiple outs or are driving it with multiple MIDI tracks. The notepad feature is cool, too.

FWIW I've had SITs not export on a full mix. IIRC it was X2 or X1.
 
I like the folder option especially for drums. I rarely use the mixer inside a drum VSTi.

I am with Sharke.  I like their convenience, and split them when I need to.
 
As has been stated, they don't work with multi out synths, so I don't use them for those.
 
I have not had problems with them not exporting in any version of Sonar.  Just lucky, I guess.

Right or wrong, I use separate audio/midi for most things, but have fallen into using instrument tracks for bass and piano.  There is typically only one bass or one piano coming from those synths, and I personally do not do a lot of exotic things to those two basic sounds.
 
I use separate tracks for drums (one MIDI track with multiple outs); RealGuitar, RealStrat, ReaLPC because I use one MIDI track for notes and one for keyswitch/strum notes; orchestral parts for the multiple outputs and separate MIDI tracks for keyswitches; etc.
 
All separate MIDI for me goes in a folder so I can hide it from my control surface: I don't want 12 faders devoted to MIDI.
 
But ask me again tomorrow...

I use a mix of simple instruments and split ones no downsides for me, can still get to all notes/automation etc. one thing that slightly annoys me about split midi tracks, if you have a plugin and you've setup several midi tracks to play it, if you mute all the midi tracks for that instrument it also mutes the audio, which I generally don't want as I like the effects tails to carry on. as a work around I add an empty midi track and hide it. should be an option to say "leave audio playing" or "mute audio with muted midi tracks" or something..would be nice.

What Morgan is saying above is an annoyance for some but a convenience for me: I like being able to solo an audio track and know that the MIDI tracks will also be soloed so I don't have to go looking through MIDI folders.
 
Depends on the individual workflow.