I agree with xstatic about small, loud stages - I mix at a club with a small stage that often features bands with really loud amps. It may not even really be that SM58s have better rejection than Sennheiser, Audix or EV "58 killers" but our monitors are rang out with 58s and since I mix 3, 4 or 15 bands a night (damn "tribute shows"!) There really isn't time to re-eq on the fly. In fact, I dont even line-check bands anymore - get up, tune up, play.
So if you want your monitors pushed, you probably are better off bringing in your own clean, not drooled on 58.
Ape