I too ride recumbents, where a mirror is pretty well essential, and then when riding an upright bike, I really miss them - so I have mirrors on those too. I like Blackburn's "Multimirror" - tough as old boots, a clear mirror that doesn't cloud over, and convex so you get a decent angle of view.
Yes, they tend to get knocked out of alignment - pretty much every time you park the bike. But it only takes a couple of seconds to tweak it back and then you're good to go.
The danger is, as they are with car drivers, they become a substitute for proper observation skills.
It's simply human nature, and unless you know the drills to avoid it you'll inevitably do the same yourself and the NET result will be no safety gain.
I agree that's a danger, but in practice I find I tend to glance in the mirror to get a broad picture of what's going on behind, to see if any companions are still with me behind, and to judge the passing distance of anything that's about to pass me. I kind of subconsciously 'get' that that view isn't sufficiently detailed to judge whether it's actually safe to manoeuvre or not, so it's still instinctive to do a shoulder check or a proper observation.
