How about straight from the horse’s mouth?
From that interview: " IGNPC: Why did you go with a primarily PvP structure? Reynir Hardarson: Well, to begin with all multiplayergames from the dawn of civilization have been PvP. So for us to create a universe with thousands of people playing together against computer controlled evil would be plain madness and missing the whole point of multiplayer gaming. We wanted to create a game where player interaction is encouraged and rewarded on all levels. The true magic of the MMOGs comes from player interaction and player competition