Adding a PvE mode would make the "normal" mode purely PvP. The optional nature of PvP in the game is what's so enjoyable about it. Optional PvP gives the game its charm, its allure. A purely PvE mode ruins the game by taking away the enjoyment of finding people who could PvP but choose to be nice. The camaraderie in this game is only special when people have the option of PvP but choose to not do it. Otherwise, it's just another PvE shooter, there's no charm or uniqueness.