The reason for this is that before the full launch we really need to test how the game worlds behave under high load (i.e when they are very full!).
Obviously we really don't want the game to start lagging horribly when there are lots of people on one world, which is why we need to do this test.
With only a few servers the beta should be much more crowded and we can see if it still works ok. So please all pile in!, and we'll hopefully get it all optimized ready for the launch.