In Dec 08 a customer at the store I work for did our survey after she purchased a $8 pack of batteries. She won our last $10,000 gift card give away! (Now the winners get $5k) She was amazed that she won since her purchase was so small. She got entertainment center (TV, speakers, etc) and a kitchen of appliances plus she got gift cards for her adult kids too.
I can't speak for other companies but mine does not sell/ use the information gathered for marketing unless you tell us to.
So take the surveys because someone has to win- why not you?