Another thing is that the police think if these stickers as a deterrent, even if they aren't, and so will make you put it in a super visible place, thereby ruining your expensive bespoke repro decals. I think most of us see them as, at best, a way to positively identify a recovered bike. In this sense, I want to put it in the least noticeable place possible, maybe somewhere where the thief won't even find it.
It also depends on what "casual theft" means. One of the most common types of theft in the uni town where I live is people "borrowing" a bike after Friday night partying, riding part way home, then ditching it in a park. A sticker will do nothing here.
For me, casual theft means anything that doesn't involve actually breaking through something, since this usually involves some kind of tool or skill/effort. Anything, even a dinky cable lock, that keeps someone from just jumping on and riding off is likely enough to stop truly casual, spur of the moment theft. Such thieves will choose the easiest target, or the most obviously valuable one, which probably means that 3000 Euro pedalec (esp. when the owner forgot to remove the controller!) and not that "10 Speed" with those weird shifty things.