Necromancer versus Light Elf.

It's easier to cast a general Dispel spell, if you think about it. By the time you kill someone to create a stack of ghosts in their place, you'll lose half your troops if your defense is insufficient.

Fairies and dancers are killed very quickly. A couple of stacks of ghosts will be enough to block the archers. And since there is anti-magic against Dispel, there is no other way (more effective) to block the archers.