I suppose it is easier to state what we DO USE, since we don't use most of what SONAR offers ..

we use ..

CONSOLE VIEW
TRACK VIEW
PRV (only if working with drum software like BFD)

That is all .. really.
We never use the other 85% of stuff.

Mostly importing tracks recorded on 24 track Analog for editing
and/or recording mostly live musicians direct to DAW.

So in effect, we really only use SONAR as a super modern MULTITRACK TAPE MACHINE with special EDITING flexibility like you can't have with tape.

Theoretically we could still be using something like SONAR 3 and be fine I imagine.

But we are using SONAR 8.3

Never had a pop, glitch or crash in 2 years.

(probably why we are sitting on the sidelines for X1 for a year or so to let the dust settle. -- with the current system running so solid and reliable, no need to tempt fate and invite trouble just because something new came out.
After all, we know all companies release something new every year to perpetuate the cashflow cycle of sales -- not because we really NEED it.
That's business !

Here's my response, with the proviso I am stll on 8.5.3
 
 
Video View     -  No, because I don't work with Video, its possible that my change in future. 
Matrix View  - I dont think I will ever find a use for it.
Beatscape  - Don't even have it  installed
Surround Sound - No use to me at the moment
Navigation view (Defender view)  - Don't even know what it is!!
Step Sequencer  - Never used it in anger yet, but I have played around with it. There is a small chance I may use it one day but I agree with Randy that PRV is far more flexable (as is the regular step time input)
Dreamstation synth - Does not work in 64 bit. Never used it anyway.
Session Drummer - any version - I would go to Superior Drummer, EZ Drummer, Steven Slate Drums or KitCore first. Session drummer is pretty good but I have better options.
TTS-1 Synth   - That I do use, mostly for sketching out but It good for what it is.
Arpeggiator - Played around with it, I had hopes for the guitar patterns but its unlikley I will ever use it in anger. Its one of those things that can provide a quick rush of instant gratification followed by the sad realisation that its not really satisfying in the long run.
Publisher - No
External Inserts - No
 
ACT - Not yet but I have recently acquired an A-Pro 800 and I will eventually.
 
Other things I don't use,
 
Drop Zone
Rapture and Z3ta - I love Pentagon however.
REX files (RPX)
Audio Snap
V-Vocal
 
Edit-
SFZ and SFZ+. I use to use them but with Kontakt, Indepencence Pro and Dimension Pro there is now no need.
 
Another Edit.
 
Cyclone.

mikespitzer, who is "we"?

ACT
Arpeggiator
Beatscape  - buggy toy, no thanks
CD Burner - no CD Text, no ISRC codes
Cyclone
Dreamstation
Loop view
Lyrics view
Matrix View - don't need until it becomes more versatile
Pentagon
Publisher - does anybody use this?
RXP
SFZ
Staff View
Surround bus
TTS-1 Synth
Video View - too limited; I use another program for video
V-Vocal pitch to MIDI - does it work?
Track Layer solo/mute
CAL language
File / Print
Import Audio CD
Import Video
Navigator
Surround panner
Insert Bank/Patch change
Insert series of controllers
Process / Remove DC Offset
Process / Fit Improvisation
Set Default Groove Clip Pitch
Set Measure Beat At Now - problematic
Window pulldown menu

Notation
Lyrics
Matrix
All of the included synths
Heck...it would be easier to say what I do use:

Track View
Piano Roll View
Console View
Synth Rack
Video View

But I've made a New Years Resolution to learn about some of X1s offerings I don't use currently. 

Course I said that last year too.

Most of the things on people's list I don't use either.  However, I don't want them gone.  Once in a while, I'm in the mood to experiment, and I want the option of using some of these features.  For example, I didn't think the step sequencer would be very useful, but I grew up on the Roland 808 drum machine, and the sequencing is very familiar to me.  I have found it to be a quick and handy way to program certain drum beats and rolls.  I can easily adjust the volume levels of the individual beats as quickly as the PRV.  I have asked for a matrix like function for years to be able to experiment with alternate arrangements, but I've only used the matrix once, and it didn't perform flawlessly.  Some of the older synths still have their uses even if they aren't supported anymore.  It's interesting to see what people use and don't use, but I hope this doesn't lead to a movement to take features out of Sonar just because a handful of forum members don't use these features.  Better to make them work as advertised.
Regards,
Lynn

These features can be removed from Sonar and I would not notice anything: ACT Arpeggiator Beatscape CD Burner Cyclone Dreamstation Grooveplayer Loop view Lyrics view Matrix View Pentagon Publisher RXP SFZ Staff View Surround TTS-1 Synth Video View

I'll just steal his list cause it it identical to mine.  I will add that as a rock guy who focuses mainly on audio, I'm not even sure what all those things are!   RXP, Beatscape?  Matrix view?  I wouldn't even know where to begin.

For me, I don't need the following:

Matrix View
Beatscape
Dreamstation synth
Many other older included synths, or LE version
Arpeggiator
Publisher
External Inserts
Notation
Lyrics
V-Vocal (Not a bad product, but I have Melodyne)
Guitar Rig - any version

Here's what I do use regularly

Track view  (including Inspector)
Console view (less and less though)
Synth Rack
Browser
Session Drummer (I usually use Reason Drums, however I do keep this around for some things)
Step Sequencer (I grew up on the old Roland drum machines - so this is a keeper to me. Combine this with the TR707,909 and 808 from Session drummer and I'm back in the 80's - LOL!)
ACT
Big Time
Pro Channel
Sonitus Effects
LP Effects (EQ and MultiBand)

Things I don't use at this time (or often) but glad it's there.
TTS-1 (I don't use if for final mixes, but it's low resource makes it great for sketchpad work)
PRV
Surround
Dim Pro
Video
Boost 11

If it isn't mentioned in any of the above lists - it could easily fall into the "I don't need it" pile.

So does anyone use a feature others have listed as useless?

Staff View and Lyric View - I think if more people looked into those, they could find them more useful than they imagine.  You don't need to be able to read music to easily type your lyrics in Staff, each syllable married to a note in a dummy melody track, then your Lyric View is ready to go, complete with "bouncing ball" kind of action with the syllables being highlighted as the project plays.  It's a useful way to have words displayed instead of having pieces of paper stuck up wherever you can in your set up.

I use these to jot down what I just played. Cause whenever I come back to the piece, I will have forgotten.