There's no doubt it's a matter of "belt and suspenders" (redundant protection).
The main reason to do it is for the installer. You are in that box later, pulling that yoke out, and either a) whoopsydaisy, it turns out that circuit was not de-energized after all; or b) you are troubleshooting a problem which "lights up" things which should be off.
Once I definitely did turn off the right breaker (the breaker was unclipped from the bus), and bumped hot against ground, got a tiny arc flash. WTH? it was a tiny flash, so I connected it firmly, and voila, a fluorescent light on the circuit lit up. These things do happen.
Also, I have had cases where due to the shape of the box, you had close contact. That is particularly when dealing with the relatively shallow steel boxes and also Romex, where you have ground wires flopping about (which you do not in EMT). Worst case: side entrance of Romex into 1-gang.
As far as thermal derate, you are at the screw terminals, where there is a mighty slug of metal. No chance of overheat there (we aren't using the backstabs, right?) If it really bothers you, do a 30% derate off the 90C numbers. (This derate is already required; natively, 12AWG is good for 30A.)