I'm thinking, wouldnt an undead be, by definition of not having a brain or even being alive, be the perfect suicide assasin/canon fodder. But, you would need a method to keep the enemy side from recycling the boys a second time. If you dont, its like playing pingpong with a massive undead horde. A sport I'm sure many necromancers would enjoy playing. Also, hig lv clerics and paladins wold be able to, literally bless the rain, killing undead left and right.
On a side note, would the ghost of a soldier still want to fight? If so, the necromancer just got the posivility of more complex tactics and strategies, thanks to these ethereal aides.