True but the truth is open source forums like that don't work on a big site. I'm a big fan of SMF as well and I use it on my site.
Although it's nice looking on the user end - Hoho! Man does it cause quite the number of problems on the admin end! There's so many security exploits, bugs, and other stuff going on that really it just makes more sense to make your own.
In short like Bill said using someone else's database and integrating it with your own = not very good in the long run. Especially on a site that you know is going to get huge.