I don't think the AI can be programmed to actually make clever choices. I think for every scenario where it makes the right choice based on such rules there's another (or more) scenario where it totally screws up because of that.

One thing i'd really like to see that'd kinda fix this problem is the possibility to script ongoing order for troops. I.e.: cast (specific soell) ad infinitum.
Or maybe less specific "only cast support/damage/summon/protection spells".