I'm a little late replying here, Nathan.
I think the less themes you offer your users, the better off you'll be when it comes to adding changes to the forum.
I'm sure you're aware of adding modifications. The package manager for SMF is a true friend...which I just recently got to work on my forum...however, some of the mods require manual edits, if there are additional themes involved...this can definitely be a pain in the rump!
That's why I don't offer any additional forum themes on the site. Less to manage and update.
I know there are those who'll complain about the loss of available themes...but, it's better in the long run.