Google are hoping to use rel=author as a ranking factor (http://www.youtube.com/watch?v=FgFb6Y-UJUI&feature=youtu.be&t=57s)
I think that in the (near) future this will start playing a role, and the attribute will become a standard like nofollow is. If you trust your freelance writers, and think they'll be able to created a trusted, authoritative profile, go for it. If you're not sure about the quality of work they deliver elsewhere, for other clients, you might want to consider creating your own profile attached to your site, and use that as the author attribute. That way you'll have a bit more control.