Yeah, I can't see that it's tricky to use or understand.
Flick it round to heated if the mirrors are steamed/frosted/covered in water. Once clear, flick the knob back round to 0. This won't take longer than 5 minutes in the worst of weathers so there's no need to leave it on once it's cleared.
That said, I've left mine on for a couple of short journeys before and I haven't noticed any sideaffects, so I wouldn't worry about it.