I know what you mean, but I can't go too overboard with my criticism as my (now dead) sister had one and she was a really nice person (well, she was to me :)
I just don't get it. If you've got a spare couple of hundred quid (or even a grand or more) that you don't know what to do with, just give it to a charity or something, or at least spend it on something that enhances your (or preferably somebody else's) life a little. A weird collection of letters and numbers on your car achieves absolutely nothing for anybody.
It's a "status symbol" that does nothing to make your life better or more enjoyable, and as you say, simply labels you as a **** to those around you.
Each to their own.