[thejeff]

The evaluation of actual damage spells seems pretty straightforward. We can quibble about whether it's better to do very likely damage to lots of little things or go for the long shot Soul Slay type kill against a high mr target, but it seems like the AI pretty much goes for max hp damage.

The usual complaints are with buffs and summons. Both of which the AI doesn't seem to treat as situation dependent. Often time is wasted casting self-buffs when the caster is well behind the lines anyway. Or summoning weak units or ones to slow to reach the fight. My feeling is that both summons and self-buffs are overweighted in the algorithm. I'm not sure what factors affect the summons calculation, but it's possible that more should be considered. Ideally range to enemy should be a factor, but at least the abilities of the unit, not just hp. This may already be done. The debug log doesn't really show you.
I would also love to see path booster spells considered. I don't think I've ever seen an unscripted Summon Earthpower and that should be a very high priority.

You could RPG it as you suggest, but I haven't noticed any real relationship. Obviously they won't cast buffs they already have, but mages are perfectly happy to Ironskin or Astral Shield if you've given them a luck amulet, for example.