Passive evasion *alone* does not a tank make, not because evasion is somehow worthless for a tank but because it doesn't increase eHP. The provision of regular absorb shields or a +hp bonus would provide all of the burst survivability needed, which WoW demonstrated pretty well with Druid tanks. Over time, increased evasion is just as effective at the role of increasing healing efficiency as DR and +healing. It's only over the short term that evasion becomes worthless because it isn't reliable.