I think that rewarding people for their hard work with a fansite item is a good trade-off. It's a program that's been working for years on many fansites.
Answering the main question, I think it can hurt or not depending on how this potential is channeled. It's true that some people post questions and answers to gain votes; that should be completely fine as long as a good quality is attained. For that, rules are being managed and adjusted.
I think there might be more adjustments to make (maybe with ideas like the one you proposed or some others). If done right, and a good balance is achieved, everyone could benefit from this rewards' program.