Here's a couple reasons I play sometimes play for free

1 - building relationships with venues,
2 - expanding your fan base,
3 - supporting a good cause (you mentioned that already),
4 - or just having some good times.

It's a tough one though. Like anything you're gonna have to watch for the signs of exploitation.



Here's a post I did on the subject... http://www.apparatus.ca/when-you-should-work-for-free/