There are times when it makes good sense to work for free.
1. You are out of work. Working as a volunteer will give you experience, contacts, and possibly a new career.
2. You want to make a film about something you truly believe in. In the digital age, media and distribution are essentially free. If you don't expect any money in return, you can make just about any film you'd like.
3. The economy sucks. The economy always sucks, someplace. If you happen to be in one of those places why not work as a volunteer for a while? Find something interesting and do it. When the economy picks up in one or two or five years you will have new skills, a portfolio and a list of accomplishments.
The trick to making this all work is to truly understand that you are working for free. Expect nothing in return. Just do the job, joyfully.