My new theory about why they put Metro in the beta is because nobody is every going to fucking play it again after seeing Caspian so they wanted to get something for the time they spent coding it.
They used it because it's a good rush map that not everyone got to see because they weren't in the Alpha. And they wanted to play-test the changes. The did make some big changes in the location of first stage M-COMS and terrain.
They should just open up Caspian to everyone now though.