I think it's fine to do it a handful of times to practice but you MUST move onto paid work fast or else you'll get stuck in free jobs. I still see people I met years ago who are still doing free work in lo/no budget productions and other than being fun, what's the point?

Let's be honest, low budget films are usually quite bad. They serve only to bring a crew together and have them practice with low risk (other than risking their own physical and mental health obviously). There are exceptions but there are always exceptions to everything.

Film making is about getting your work out to an audience, and that audience has to be able to finance your work. If that isn't happening then something is very wrong somewhere.