It's not like builds would mess up the DF either. Take Paladin for example, you give it a core of tank skills that it always has but then you can build it's optional stuff around say shields, heals, debuffs, buffs, damage, etc.

It's a tank, it's tank kit is balanced to perfection, it's skill trees are not mathed out because which build you want will change depending on the other 7 people in your group.