Q. Why Beta? A. We need to get some data into the system before it will be fully optimal. That means the initial categorization of items might be a bit off until we see what kind of tags are becoming prevalent and tune the system for the best results.
Translation: "We want to get some tags in the system so when it goes live it'll be immediately useful and we can delete the stupid tags"
Right now, the worst thing about tagging is that you don't even have to own the game to tag it.
Edit: People are apparently also using tags to put spoilers in for certain games.