I never once was in a life-threatening emergency that I would truly have to remove my rig underwater to resolve.
However, there were some times I did it as a matter of convenience:
* I went diving in a rashguard with my harness still adjusted for a drysuit, so I took it off to loosen the webbing, then adjusted it better. Spent the safety stop fiddling to get it just right.
* I got a bit of a tangle on abandoned line. It was good line, so I half-removed my gear to untangle it and make sure I recover all of the line and not just one end.
* When diving in the Med off a smaller yacht, it was far safer to don and doff my gear in the water than to endanger myself climbing the ladder in it.
Plus, a lot of times mostly for fun:
* Since you never have to pass through restrictions, does that count? Yes, SM is best for it, but sometimes you just aren't in a SM rig.
* A few times when I wasn't dedicated-diving, I'd doff my gear, pass it to someone else to carry back, and switch to whatever water toy they brought me.
* Lots of times just for practice.