In the beginning when you start off a creative career of some sort you might take that opportunity that pays absolutely nothing to get your name out there. A few is fine, I mean I’ve done my fair share of free work. However, once you are established or been around the block a bit there should be no way people are still asking for favours in terms of exposure.
In fact, I’d say the second that you learn how to make money from doing what you love is when you stop accepting to do it for free. If you’re doing it for free, you might as well be doing it for yourself and retain all your rights. Post it on your own blogs, portfolios etc but don’t let people take advantage of you on something that they totally have the means to pay you for.
Anyways, exposure is poop, it doesn’t pay rent.