Since RPing is my hobby rather than my livelihood, I might do it for free if it were for a friend. Like Trancejeremy, I'd like to have a copy of the publication, and might even do it for that reason.
If I were in a field where being published in RPG products would further my career, then I'd certainly do it for free, if I thought the long term pay-off would be worth it. But unless one were a professional writer, I don't think being credited in RPG products would be a good resume pad. In fact, given how some people feel about RPGs it might even backfire. I know that, were I to get something published in an RPG source, I wouldn't include it in my list of publications on my resume. For one thing, I'm not especially interested in chatting about my hobbies with potential employers. It falls into the none-of-their-business category, like the rest of one's personal life.