I'm not a pro magician, but I am gaining extensive performance experience at school every day. And although I often use gimmicks (right now, anyway), it totally builds my confidence, and allows me to develop the social skills and ability to -perform- a magic trick.
I do tricks with sponge balls, regular decks and coins, all obviously involving basic sleight of hand. And, oddly enough, I get the BEST reactions with probably one of the simplest routines I know, and that is with sponge balls. After seeing someone start screaming and jump off the ground when they open their hand to find two blue balls in it, I knew right away that the hours of practice to totally perfect the sleight of hand to do it right every time was worth it.
In my own opinion, after getting back into the magic scene just this last month (I did it more when I was younger), gimmicks are invaluable for developing performance skills. I know enough about entertainment to say that the simplest trick with a good performance makes the best trick. Any day. The most complex sleights are worthless if you don't PERFORM them! Doing tricks with 'dime and penny' CAN blow people away--if the performance is good. Otherwise, forget it--try doing a good little trick like that to someone without saying anything or building it up at all. It will lose most of its power.
I don't mean to step on anybody's toes, I'm just voicing my opinion on this. I do think sleights should be mastered by every magician. But that doesn't mean gimmicks are worthless; they are valuable crutches for the the magician who lacks good social or performance skills. Do a trick that you just CAN'T mess up on to a bunch of strangers, and you'll gain some confidence and learn a few things about performance!
I've spent quite a bit of money (for me) on gimmcicks and other magic supplies (including decks, hankies, gimmick decks, coins, gimmick coins, etc...) as well as a couple good books. I don't see myself buying any more gimmicks anytime soon, but rather purely educational matieral for the purposes of developing skill in sleight of hand and performance.
Anyways, yeah--I'm tired, just thought I'd say something... again, not challenging anything anyone said, just putting in my two cents..