An off-the-shelf product will never let you customize nearly as much as when you roll your own. The whole reason we're having "Forum Feedback Days" right now, is because this forum is custom-made and so the owner has the *ability* to tailor it *specifically* to the wishes of the membership.
That, plus it's likely faster than anything off the shelf. If I remember right it runs on Perl and text files, no SQL database. WW is one of the *largest* forums on the planet. That kind of size doesn't always scale well.