It's certainly a bit of a conundrum, last season I forgot to apply it once came very close to a CL defeat.
The season before at L4 left it off in one cup game for the last 16 1st leg, only defeat upto that point.
In my case I've used them so often it's a easy excuse if you get beat and don't have it on.
Bit like swimming with a life belt, you know you can swim but might drown if you push it to far without it. lol