Dudes, yall are thinking too strictly in terms of "AI" and "attacking" a stack of summoned illusions makes the best defense .... evarr. Idk if the code already makes illusions defend first or not ... but they HEAL 100% AFTER COMBAT!!! this is great when attacking people that use hordes of weak infantry (in MP) like say, the orcs. The only way to kill an illusion is to defeat it in 1v1 combat. So u need powerful units with high percentage victory to attack them, otherwise you are losing units left and right.... just giving them the advantage flat out. Assuming u have killed all the illusions, most likely that was your "good forces" so you may have a hero or two plus your cannon fodder to take out the real army.
On offense or defense, illusionists work great with assasins, and work well with other unit types as well. Probably why the svartalphar get them, to compliment their strong recon line. Anyways, this strength of the illusion is often lost among sp players due to the AI's bad tactics imho.