Ha! By "note", you didn't mean notes, you meant NOTES. I thought you were talking about notes as in "make a note of that", but you meant notes that are played on, say, a flugelhorn. 
 
Still, it's a potentially useful idea. I can think immediately of having a muted track, then recording to it when auditioning a version of a piece, plopping down a note every time I hear something that needs fixing.

jkoseattle
I guess another reason for keeping it chopped up was to be able to export versions and hear them away from the computer to take notes.

You can use the Export feature, you can select among Tracks or Buses, from Time to Time (based on Markers or not).
So you can handle all your piece into one project and then export what you want.
 

jkoseattle
I'm surprised the vast majority of you are saying to keep it all as one huge file, but I admit that does sound appealing. I think I'm worried about navigation mostly. Markers are such a limited feature. I would want to use them both to mark whole sections, and I also use them when I'm scoring a bunch of instruments over a complex chord progression, I make a marker for each chord change. I wish they'd give us multiple levels of markers for that. 

For the chord progression, simply use a dummy track that will contains multiple clips with no event inside, and named each clip with your chords. This may take a long time, but you can use magnifier at measure time. Don't forget at the end to lock all the clips (no move, no edit). The drawback is that if you zoom out too much clip names would overlapped or disappears, but it would be the same with a dedicated "chord track" with too close chords.
 
If you mix multiple instruments for the same stave, you can group them into a folder to organize your 20 instruments with a sub-level.
Another solution is to put many take lanes on the same track and activate all track lanes for layering, but I think it would be more confused as you'll have to drag'n'drop from a standard track to the target track lane.
 
Hope this help.

Phenaste
 
For the chord progression, simply use a dummy track that will contains multiple clips with no event inside, and named each clip with your chords. 

 Since I spend a majority of time in PRV, I don't think this solution is going to work out.
 

Phenaste
If you mix multiple instruments for the same stave, you can group them into a folder to organize your 20 instruments with a sub-level.

I already do that, and in fact I will open PRV for a section, then lock it, open another section, lock that, etc., so I have a tab for each section's PRV. If I could name or save those PRV configurations it would be even better, but this works good enough.

I routinely record live concerts of over an hour in length using two Tascam 16-channel interfaces = 28 audio channels going down on Sonar 8 running on a Netbook XP 32-bit with only 2GB RAM.
 
Anything on a modern PC should be a cakewalk by comparison.

OK, thank you everyone! I have a working solution now, based on suggestions here. Glad I asked.
 
I have created a muted dummy track at the top of the project (Track 1), and entered a single Midi note at the beginning of each section. Each Midi note in this track resides in its own clip. I then drag each clip out to the end of its section, name it for that section, and give it a distinct color. Now, I can easily see each section from that dummy track's colored and named clips, and can scoot forward and backward by section with the Tab/Shift-Tab keys. Works great! What's also great about this solution is that I'm not using Markers at all to mark sections, so am free to continue to use Markers for individual cues within sections. 
 
So OK, I'm going to next combine all my separate projects into one big file. In this case, I'm trusting all of you that I'm not going to massively regret that decision :-).

New problem:
 
I have to do a lot of vertical scrolling because of the number of instruments. So the dummy track with the section clips scrolls off the page a lot. To solve that, I decided to make a clone of the dummy track and place it near the bottom so that I can always see dummy section clips no matter where I am.
 
I selected my dummy track, right-clicked and chose Clone Track. In the Clone Track dialog I checked "Link to Original Clips", but nothing is actually linked. It just makes a copy of the track. I tried entering new notes, dragging clips around, clearly they aren't linked. Plus, the supposedly linked clips don't have that border around them.
 
I then tried simply creating a second dummy track, (not cloning) and copying clips individually, indicating they should be linked. This also doesn't create a linked clip, so it seems that the problem is with linking itself. Why aren't my copied clips linking? Am I missing something? 

jkoseattle
New problem:
 
I have to do a lot of vertical scrolling because of the number of instruments. So the dummy track with the section clips scrolls off the page a lot. To solve that, I decided to make a clone of the dummy track and place it near the bottom so that I can always see dummy section clips no matter where I am.
 
I selected my dummy track, right-clicked and chose Clone Track. In the Clone Track dialog I checked "Link to Original Clips", but nothing is actually linked. It just makes a copy of the track. I tried entering new notes, dragging clips around, clearly they aren't linked. Plus, the supposedly linked clips don't have that border around them.
 
I then tried simply creating a second dummy track, (not cloning) and copying clips individually, indicating they should be linked. This also doesn't create a linked clip, so it seems that the problem is with linking itself. Why aren't my copied clips linking? Am I missing something? 

Maybe Track Folders will help with the vertical scrolling?
If you're using a lot of MIDI tracks to feed a few Synths, perhaps create a Track Folder for each major section of the project, and in each folder create a MIDI track that only submits data for that section.
You can have multiple MIDI tracks feeding the same Synth/Channel...

Thanks, I have one Midi track for one synth, not sure of a use case where I'd have multiple tracks feeding a single synth.
 
I do use track folders, but I'm scanning up and down between sections so much that I just leave them open all the time. I use them mostly for muting and soloing, and also for selecting tracks to see in PRV.

jkoseattle
Thanks, I have one Midi track for one synth, not sure of a use case where I'd have multiple tracks feeding a single synth.
 
I do use track folders, but I'm scanning up and down between sections so much that I just leave them open all the time. I use them mostly for muting and soloing, and also for selecting tracks to see in PRV.

For a stripped down example, you've got 3 musical sections (verse, chorus, bridge, for eg) to the project, with a trumpet, double bass, and violin playing throughout.  
Create a Track Folder for Verse, with 3 MIDI tracks in it.  Trumpet Verse, Bass Verse, and Violin Verse.
Create another Track Folder for Chorus, clone the Verse 3 MIDI tracks, change the names, and put them in the Chorus Folder:  Trumpet Chorus, Bass Chorus, and Violin Chorus.
Create a third Track Folder for Bridge, clone the Verse 3 MIDI tracks, change the names, and put them in the Bridge Folder:  Trumpet Bridge, Bass Bridge, and Violin Bridge.
All MIDI tracks route to the synth you're using to create the sounds (all MIDI Trumpet tracks have identical settings, feeding the same channel, etc), and the Synth Outs don't need to be in these track folders.
 
When you're programming the first part, you can close Track Folders B & C, and have all the MIDI tracks visible for the first part.  Then, when you move on to the 2nd part, you close Track Folder A and open Track Folder B.  Again, all MIDI tracks are visible.  All instruments will keep playing as programmed, but you're able to focus visually on the places you're working on.
 
Does that make sense?  
It's obviously not needed for a simple 3-part, 3 instrument project, but if you're getting overwhelmed with lots of tracks and parts, it should help to organize it.
 

And would screensets not maintain open/closed state of folders, in which case a quick key press could switch between the "verses"?  Not at SONAR to check this out right now...