An illustration is in order. When I first moved to LA, optimism flowing buoyantly across my synapses, I worked an unpaid weekend short film. I blithely thought that this would net me some connections that would prove valuable in the future, possibly even extending to a real job for the same producers.
This is an example of the first snare laid for you by the offerers of unpaid work: their gratitude. Witness the conversation I had on set with the producer:
ME: I'm super-psyched to be working this free gig for you!
PRODUCER: You're welcome. And we'll totally hire you next time for real money, too.
ME: Really?!
PRODUCER: Oh, totally. DIRECTOR and I are line producers for music videos. We'll hire you for the next one we do.
ME: Awesome!
This three-day shoot included some of the longest hours I have ever worked on any job before or since. First day: 12 hours. Fine. Second day: 18 hours. Third day: 20 hours.
No doubt this was related to the fact that these people worked in the music video world, which is notorious for its abusively long hours. Regardless, this illustrates the second snare of unpaid work: mistreatment.
See, although you'd think people would be so grateful that you're working for them for free that they would treat you as your enthusiasm deserves, in reality, your willingness to accept their thanks as payment instead of money just signals to them that you are WORTHLESS.
When someone pays $200 a day for PAs, they know they need good PAs, which means they damn well better treat you right, or you won't work for them again. And while PAs are a dime a dozen, GOOD PAs are much more like brilliant TV executives*
But the final snare, the third reason not to keep working for free, is that it DOESN'T WORK.
Recall PRODUCER'S promise to hire me on her next shoot. Then CUT TO: Three months later. I get a phone call on a Saturday night:
PRODUCER: Hey Ryan, it's me, PRODUCER.
ME: Oh yeah ... wonderful to hear your voice.
PRODUCER: You know it. Anyway, I was hoping you could do me a favor. I need another PA on a job tomorrow. It'd pay a little more than last time.
ME: Yeah?
PRODUCER: Yeah ... how does $50 sound to you?
ME: Well, it sounds terrible. But the wife and I are pretty hungry right about now ... so okay!
In other words, people are never going to give you real money if they know you'll work for thanks. Don't let them keep taking advantage of you. And you're unlikely to get any real connections out of it, so only do it if it's good experience (when you're starting out, any experience is good experience).
To sum up: it may be okay to work an unpaid gig for someone one time, but if that person calls you back for another shitty-paying gig ... RUN AWAY.
*Just my bitterness talking. TV executives have to be brilliant, otherwise how could TV keep making all that money?