Compared to what? Incinerate? Yes, incinerate deals a truckload of damage, but then again ask all those sorcerers that used in in D3 and how they whine about getting instant aggro and stuff ...
20 dps? Whats the problem with that? You got 2 of them, they got a lot of armor penetration so they always deal the damage they should ... this is like a perma-damage debuff like crippling curse. And plus, it doesn't add to your threat. What do you expect? Should the skeletons deal as much damage as a whole hero class when you still got all those magician spells to back you up? I think that wouldn't be fair.
besides: the skeletons add something to your utility that sorcerers will always miss: physical damage that ignores resistances.