Diablo 2 had one.
WoW had one.
Starcraft 2 will have one.

Each one had a game online structure to test, and the more players online, the more worthwhile the test.

But they came after a closed beta, were limited in scope, and had a short duration. Diablo 2 had zones up to Blood Raven, and (correct me if I'm wrong) WoW had a level cap of 40.

Please use existing threads to discuss this.