If we're talking D&D type zombies then raising them as undead totally *does* harm the person they used to be, because you render it impossible for that person to be raised from the dead until the zombie is destroyed. If we're talking a bit more real-world than that, then I question the usefulness of this scenario anyway--so, you raise Grandma's rotting corpse and send her against the enemy army? Not sure exactly what an unarmed woman would do against armed soldiers even if they were in tip-top condition, but in this scenario I think she gets chopped into rotting chunks before she gets close enough to hurt anyone.

But, anyway, let's ignore all that and assume that your undead defence actually works and you chase off the enemy. Just how expensive is this going to be? If a necromancer is literally capable of producing an effective army from the local graveyard in a matter of hours, what's to stop him turning them against you and just taking your kingdom for himself, on the assumption he'll make more money looting the treasury than you could ever afford to pay him? Heck, what's to stop him selling his services to your bigger, presumably richer enemy and sending *their* undead hordes in so they don't have to sacrifice any of their sons? After all, we have to assume that someone who is willing to rob graves and resurrect the contents probably doesn't have an enormous amount of moral integrity.