There is a new branch in portal repo called 'buffs'. It is a try to implement sane buffs for bots. New buff() method performs several checks before casting a spell which include:
1) Target level. If level is too low, then AI looks for a lower rank of the spell.
2) Target auras, If spell applies an aura which is better then one that already exists on target, or target does not have such aura at all, then spell is casted. This check allows to overwrite auras from same spell if new aura has better modifier. One example is talented Blessing of Might. MaNGOS core does not perform this check and always overwrites such auras.
So far only paladin AI was updated to use this method. There is one flaw in current implementation, I want to point to. Since paladins can have only one blessing per target, in several scenarios paladin bots will "lock" their talented spells for buff targets. One way to solve this problem would be implementing buff manager, which will decide how each bot buffs each target. This will require some kind of command queue for bots.