For all of you masters of the English language who are trying to spell out what Massively Multiplayer Online means, I've got a revelation for you. No where in those words is there an indication of cooperation. Multiplayer does not equate to teaming up with other players. That's just the direction the MMORPG genre has taken over the past 15 or so years. Think back to original Ultima Online, sure there were benefits to teaming up with others to fight harder content or gank some poor noob. However, you could also go it alone, completely rogue.

I'll give one more example: Imagine someone made a Running Man style MMO game where a thousand players are trying to get to the end of a gauntlet dungeon, gladiator-style, every man for himself. This would certainly fit the criteria of Massively Multiplayer and Online, but there would be no need for a party system for it to fit that criterion.