In the course of your career as a musician or artist or any sort of freelancer you will be asked to provide your services for free. And it almost always comes in the form of "exposure". The question is do you take the gig or not?
A lot of my musician friends will say "Never work for free". And on the surface I agree with that statement. But only because there really is no such thing as "free".
Even if you don't get paid for a gig then there's still an exchange of something. That something usually comes in the form of exposure or experience. Knowing these 4 categories and where you're at in your career can help you decide whether or not to take that "free" gig.
Are you:
1. An experienced musician with a large fan base and good income?
2. An experienced musician with a large fan base but low income?
3. An inexperienced musician with no fan base but have a trust fund? (I can't see how inexperience and large fan base could go together)
4. An inexperienced musician with no fan base and little or no income?
For number 1: There's really not a lot of incentive to perform for free here unless the exchange is exposure in the form of playing at the SuperBowl. (The Superbowl doesn't pay their musicians because they offer such massive exposure. Of course if you are big enough to play the Super Bowl do you really need the exposure?)
For number: 2: If you already have a large fan base then exposure, again, doesn't mean a lot and you don't really need the experience.
For number 3: You may not need the money but you could surely use the experience and possibly get a fan or twelve from the gig. Go for it!
For number 4: You need everything so say yes to every gig at this stage.
Of course, there's a 5th category which is the service gig. It's where you offer your performance in the name of service with no strings attached or want for anything in return. To a lot of my spiritually-minded musicians be mindful with this category as it can kill a career if not balanced right.
In the end, it really depends on where you're at and what the exchange is. In any given interaction between yourself and a client there's always the question to ask "Is this worth it?" And again, here, "worth" can mean many things. So be clear on your current definition of "worth" at the stage you're at in your career.