I suppose if you think about it, it makes an element of sense to keep a population of characters that were built *for* the test server, as it lets them trouble shoot the current builds more effectively based on a completely isolated, build-specific data.

Copying characters from the main server might bring with it 'legacy' issues from Pre-F2P, depending on what those characters have done over their lifetimes.

...Of course, I'm talking out my butt here to some extent. Just speculating.