My take on it is that you have a sliding scale of content difficulty you can complete, where the upper bounds are limited by the tank's tankiness, the healer's output and mana regen, the DPS output, etc. For example, if you have 5x DPS (like a 5-hunter or 5-warlock team in retail) then you can burn some things down before they eat the pets, and the limit on the things you can burn down is usually limited by gear. So you don't necessarily *need* a tank, but it does help make survivability in some situations go up quite a bit due to the incoming damage mitigation and threat management that tank specs bring.