EVE Online do this. It's great.
To avoid loss of revenues because people might be tempted to play only on lam reset the server and characters every two weeks. During official tests, do not reset the server; if the server were up all the time, I would probably prepare some characters for the new content.
When Lam most recently came up, I did the things that were my priority: testing a couple of build ideas. When I finally got around to thinking about the new quests, the server was down again. Ditto last time.
There are even some potential marketing benefits:
- P2P or premium players can sample packs
- P2P or premium players can sample items, upgrades etc.
- New players can practice harder raids (eg. old abbot), and may realize how fun they are when you know them
- People can see how nice some of the pack-related gear is, and may buy the packs.
Just my 0.02c worth, but access to a test server seems to me to be a two-way street, not a favour. Maybe I'm wrong.