Necropolis discussions.

It seems he only attacks when his summoned creatures block him from all sides, meaning there is no escape, or sometimes when he cannot reach the regular units, he attacks the summoned ones.

Regarding the first point – yes, if there is no escape. But if he doesn't have the opportunity to reach a non-summoned stack, but he does have the opportunity to attack it, the AI will move closer, but it won't attack the summoned creatures. If he has no opportunity to attack non-summoned units at all, then he will attack the summoned ones. Sometimes, this can be very advantageous.