If you buy a license while it is in beta, your license covers all future versions of the game when it reaches the full version. Others have covered that in this thread. Furthermore though, the price rises with most major version releases, so the longer you wait the more it will cost. If you buy a license now, it will be cheaper than the final price.
Finally, even though it is a beta version, it is very much playable and enjoyable (as you would see if you played it for a while) so it is entirely worth the small cost.
If those reasons aren't good enough for you, I don't know what else to say.
Locking this topic as it has been covered before, and I can see nothing useful being added at this point.