It's just one of those stupid "fashion" statement things that people crap on with. Some people might think you're not "a real biker" if you have one. Personally, I think things like ettiquette and fashion are evil and disgusting, and I try to avoid their deceptive practices so I can join the forces of good (i.e. individuality, doing what I want).

As far as a mirror goes, I had one, but I didn't find it really all that useful. When I wanted to see what was behind me, I always ended up having a quick glance over my shoulder instead. About the only time I ever noticed it was when the sun was behind me, and then not in a positive way. As a result, when it broke off on a tour a few years back, I didn't bother to fix it.