I've been lucky in the respect that, as an intern, I've always been paid. It wasn't what could be considered a living wage, but considering where I was working and who I was learning from, the experiences and contacts were worth it. Would I do it again for free? For one production at an amazing theatre working with the right people - yes, I would. But I would have saved up prior to taking the position

I don't know about the internship models now, but when I worked at NSMT before it closed (and reopened) the majority of the production team was comprised of interns, some in college, some fresh out of high school. We were treated very well, we attended workshops to help improve and further our careers, we worked with highly experienced designers, etc. who were always willing to answer questions or otherwise impart their wisdom...We all knew that a large part of the full-time administrative and production employees had begun as interns, so it was reassuring and motivating to know that if you did good work, rewards came soon after.
If someone were to consider an internship for little to no money, they should first find someone who recently did the internship and asked how they liked the experience and how they were treated as an intern. Not all programs are the same!
As for working for free outside of an internship - working locally on a contractual basis, I've always charged a fee for SM work. One theatre here does spectacular technical work but doesn't pay their actors or production team, and they've asked me often to SM for them. I've been tempted, but I have to think as a business person: why would I offer the same services to one company that refuses to pay, when another company is willing to pay me? If the company that pays me finds I'm doing the same high-quality work for free somewhere else, what motivation do they have to hire me back for a fee? Or be willing to secure me at a higher rate when I have more than one offer? Plus, I find that it creates competition - the old adage that what everyone can't have, everyone wants. Smart companies AND smart directors know that a bad SM can ruin a production and are willing to pay to make sure that doesn't happen.