There should be another option in the poll, I think:
I use neither tabs nor the sidebar—I use only goto anything (and cmd+option+up). I used to use just the sidebar, for the same reason that Jon does (i.e. the fact that I never close files so tab titles are unreadable) but I realized over time that I never actually looked at the sidebar, and only ever used goto anything to open a file. Eventually I just turned the sidebar off.
I save files all the time, so for me there's no functional difference between a file being open and being closed, save for the undo history. (I wouldn't say no to a persistent undo history though!
That said, there are two reasons I turn on tabs (both of which are things that could be fixed):
• I turn on tabs when I use a multi-row layout, because otherwise there's no border between the sections (I'm using soda theme—haven't tried with the normal theme).
• I turn on tabs in order to pull a view out of the window, because you can't pull the sidebar items out into a new window.
I don't think there's a key combo either.
One other thing that would be totally sweet: If I just have one folder open in an unsaved project (i.e. I "subl ."ed something) could the folder name be displayed where the project name is? This would give me yet another reason to be able to leave the sidebar off.