Well, the divine branches are usually useful, particularly the Fortitude branch with the Healing spell. And the other brnches combine quite well, Criplle Enemies reduces an enemies crushing resistance while Divine Might increases the amount of crushing damage that you deal.
You like the AI? Heh, I can't stand it. Whenever a follower/summon sees an enemy this rush straight into it, usually starting a war and getting themself and you killed.
I think that the enemies often attack the first person they see, that's how they would attack the caster rather than the summon. I suppose it compensates fo the lack of tactics that all the followers suffer from.