I think there was value from backlinks to the designer's site in the past. In a post-Panda world, you would have to be out of your mind for putting a dofollow link back to your site in a theme footer with no control over where the theme is used. I have read about some designers who are frantically trying to identify all of the sites using their theme so that they can neuter the footer link with a nofollow. There is still promotional value when someone follows a link back to the designer's site. A good theme can drive business that way, especially if they develop custom themes.
I would never consider using any theme with an encoded footer or one containing links that led anywhere except the designer's site.
Almost every free theme that I have tested was horribly coded and lacked commonly used features found in the WordPress team's TwentyTen or TwentyEleven themes. When someone brings me a theme that they like, I typically convert it using TwentyTen and HTML 5, rather than struggle with the problems I see with so many free themes.
I frequently remind people that there are no qualifications for anyone who develops a WordPress theme or plugin.
"Democracy is two wolves and a lamb voting on what to have for lunch. Liberty is a well-armed lamb contesting the vote." -- Benjamin Franklin