First off -- I DO use the PTS. That doesn't answer the full range of reasons for asking.
PTS is one answer to a request for more character slots. It solves one facet of the request, but only that one facet.
Neither that nor playing on EU lets you have "roleplay specific" toons, it doesn't give you the extra storage space, farming capacity, or ability to play on level with friends as they start to play (the latter without deleting toons, and that's capped).
Why wouldn't giving those who pay the recurring monthly fee a couple more slots make business and player sense?