OK, so someone at Sony Online Entertainment had a fairly clever idea: Release 5 of its games for $39.99, with one month's worth of play in each of them, as a way to hook new players.
This is pretty clever, as the revenue model for these games is largely based off of continuing subscriptions, yet a player can usually only play one MMO at a time. Also, as Joystick points out,
The variety of MMOs on the market can be confusing to new players, especially if you want to try multiple games out before you subscribe. Sony appear to have acknowledged this need with their new Station Access Collection which includes Sony Online Entertainment's top MMOs in one package.
Of course, the free accounts offered on games like Planetside, and free trials on games like EVE are also good ways for people to try out a game, and it's notable that its SOE doing this and not Blizzard. Still, if I was in the market for an MMO that wasn't World of Warcraft, this box set would be a pretty appealing way of testing out different game experiences, and a good way for SOE to hook any customers who haven't already been captured by World of Warcraft or one of NCsoft's games.
Update: Through Penny Arcade, I learned that SOE is giving away the Everquest II client to Fileplanet members. The 5 client release is clearly part of a coordinated strategy to increase their customer base, either to rake in subscriptions or to lay the foundation for alternate revenue streams (as csven suggests in comments).
I suspect there's more to this than simply hooking subscribers. After all, Sony has indicated an interest in moving toward a different business model; the subscription revenue model you mention may be gone in a year or two.