I would have thought having them about as powerful as soldiers would be more
appropriate. Even then its pretty marginal unless you are skint.
Having them not decay if maintained by the necromacer by payment of Aura (say 1
Aura/50 zombies/month) would be more interesting. It places a limit on the
number you can have, and means that if you kill the necromancer, you kill the
zombies.
With that ability to maintain, it would even be worth training the zombies
(which I think should be possible) to get cheap but effective troops.
Perhaps
zombie soldier (5,3) --> zombie pikeman (5,15)
--> zombie swordsman (15,9)
zombies of course cannot ride horses
[aside - there should really be a man-at-arms type warrior made from swordsmen
by adding armour (probably chain rather than plate).]
Just my two bits.
John