Nowhere is there a written rule or law that stipulates that an MMO absolutely cannot have content within it that doesn't necessarily require more than one person to complete. If I were to log onto a MMO and there be nothing that I could piddle around with to pass the time until raid time and/or friends log on or whatever...well, I'm not going to stay on long either.
MMOs definitely are a prime game for having a plethora of multiplayer content, though.
- - - Updated - - -
WoW patch 6.3: Presenting Air Fields, aka Shipyard 2.0, aka follower table 3.0!
You think you don't, but you do.