I think it depends on the individual writer. Some do it for a career, in which case, I can't see any advantage to them writing for free. Some do it as a hobby because they enjoy it. I fit into the latter category. I have received payment for my writing, but it's pretty much a pittance. Should I ever feel motivated enough to try and make writing a full-time career, then I would definitely give up writing for free. But I do it mostly because I love it.
