The beta is because the stable release is still SMF 1.1 and the documentation on the wiki isn't complete nor is it entirely correct. For the most part, it is, but we don't want to lead people to believe that this is everything or that the documentation is always going to be correct. It is kind of like Google's use of beta tags. GMail has had a beta tag since it's release (you can still turn it on beta with additional features) and it will probably never get out of beta.
SMF's documentation will be out of beta at some point, but we aren't there yet.