Maybe because of folks like you?
Seriously, I hear you--the SM57 is a workhorse, tried and true--I know, I've got 3 of 'em. I've also got three of the "knockoffs" they're talking about. In fact, I'm the guy that did the rather extensive shootout that solaris mentioned. (By the way--it's a shootout of 6 different mics, including a ribbon--so it might be interesting to you even if you're not interested in the "other" 57s.)
Bottom line is this: I like 'em both. I use 'em both. They're not identical--the midrange bump in the knockoff isn't in the same exact place. I've got over 70 mics--from really cheap to really expensive (for me anyway), and knowing what I know right now--if I had $100 in my pocket and needed to mic some cabs or toms, I'd by three of the ES57s instead of one SM57. (My three "knockoffs" are always in use on my house drum kit.)
Sometimes I know it's smart to buy the real deal and not screw around with imitations. But sometimes I feel a little bit smarter when I beat the game and win with a killer deal.
So by all means, start with a "real" 57--you need to see for yourself what the legend is all about. I'm sure you won't be disappointed. But when you need three more so you can leave a bunch in place on your toms or multiple guitar cabs--be dangerous and try the ES57s. You won't be disappointed with them either.
And if you are, look me up. I'll buy 'em from ya!