Isn't the purpose of a tank to mitigate damage for the rest of the group, whereas DPS is to do as much damage as possible. If everyone is DPS that means you would have tanks specing gear differently to get that max damage and therefore it comes down to the mob attacking whoever does the most damage which maybe some DPS with low vitality. I like the fact that their are tanks in this MMO, don't change a format if it works. Guild Wars 2 just wants to be different for the sake of being different