I still use this old guy:
I'm not really a better player for playing stick, but I am a more versatile one. I bought a stick because it's the universal control method for fighting games. By being able to play on a stick, I can throw down in arcades and I am able to play on any platform without having to worry about different gamepads. I've most definitely made uses of this ability.
Depending on the game, I can play just as well on a gamepad. I can play BB and Tekken on pad and stick equally well, and I'm not that much worse off when I play Melty on pad. I can only barely play SFIV on a pad, but that's pretty much the only fighting game I struggle with without a stick. Plinking and double-tapping help a lot in SFIV, and those are almost stick exclusive techniques. Despite this, there still exists some very strong SFIV pad players.
If you want to get a stick, buy one because you think it'll be fun to play on (it's certainly fun for me), or because you want to be able to play fighting games wherever you find them. I wouldn't recommend that you purchase one mainly to improve your play.