[Aerich]

To me, mages are good for three things:

1) damaging or killing large numbers of enemies with one shot; their biggest advantage over any other class. They have more mass-damaging spells available to them.

2) Single-target killing; a mage is often the key to taking down a tough caster - Acid Arrow, Chromatic Orb, Flame Arrow, Magic Missile, etc. I use MM almost exclusively for pulling down Mirror Image. These spells can quickly disrupt and/or disable a caster, in addition to their damage potential.

3) Combat enhancement; they have spells to buff up your troops, as well as crimp the opposition - two of my favourite spells are Emotion:Courage and Web.

I agree with you in the sense that the game can be completed without mages, and they often seem under-utilized in capable parties, but they can be real killers.