I had a “hide sidebar” bookmarklet for Reddit that lived in my favorites bar and got used dozens of times a day. I didn’t mind so much when Reddit was fullscreen, but when I was cramming it in with other windows on my desktop that extra real estate made a huge difference.

Personally I see the value of the sidebar but I think it would work better as a fly-out UI element that can be reduced or hidden, and stays that way until opened up again. I’m in the process of trying to stand up my own instance to see how that could be done.