Tanks are needed because of their rarity and the need to have one to satisfy the DF requirement. They're not needed for their expertise at all. Most dungeons in this game can be tanked by a trained monkey just fine as long as the rest of the group isn't consisted of trained monkeys. Most tanks in DF are terrible players but it is not worth the effort to cry about it unless you're also a terrible player. The tank, in terms of dungeons, plays a marginalized role that has very little bearing to the group's success. As long as the tank isn't outright terrible his impact to winning or losing is minimal. For example on the Flan boss in WP I had a tank who never picked up any adds. After wiping once since we expected the tank to get aggro, we just had the healer ran into the flan and AE around the healer instead (since healer always gets aggro on all the adds) and didn't miss anything.