I have been playing around with the Ash Spawn summon and I must say I am impressed with just how much damage it is doing. From what I can tell, it seems to be much more effective than a frost atronach. It can hit at range as well as melee, and its dps seems to be greater than a fost atronach since it melees like a weapon wielder, rather than the rather slow punches the frost atronach uses.
In fact, I am thinking that the Ash Spawn may be more versatile than a dremora as an all purpose summon. Unlike a dremora, an ash spawn is not limited to melee combat. Therefore, in a dragon fight, the ash spawn can attack both while the dragon is airborne and while it is on the ground.
Overall, I am rather impressed with the utility of this mid-level spell.