I think writing for free is a fantastic idea in general, because anything you create is of value as a reflection of your talents. If you write a superb article and give it for free to your local newspaper, and it clearly links to your website, you could get tons of requests from other groups that read it for you to write for them too. If you can write really well, that skill will be in demand.
Also, as long as you own rights to your content, it is time well spent. Say you spend 2 years writing a lot of content on freshwater aquariums. At the end of 2 years you now have an entire book's worth of material on the subject. Even if other people ran it for free during that time in magazines and such, you now have finished your book AND you have gotten a ton of free publicity for yourself in the meantime. So when people see the book in the bookstore they'll go "Oh I know her! I loved her advice! I'll get this book by her!"
So it all comes down to quality. If you write well, the more of your content that is seen, the more people will actively want to get everything else you've written. It's like say a Stephen King book. If someone reads ONE book for free at the library, they're very likely to go buy the rest because they love his quality. But if they don't even read any of his content, how will they know he is a quality writer? Trusting your friends who read it is one way, but it means your friends will have to have read him. So that free quality sample is critical in getting the snowball rolling <img src="/images/graemlins/smile.gif" alt="" />
I know many, many writers who began by posting content for free on websites. That led to a loyal following, which led to magazine offers and book offers.