If there existed an AI that could do everything I wanted to do with a stack of units depending on the circumstances, that would be the case; there does not, that I know of. Even with something as simple as the Civ 3 model, there's the issue of trying to wound enemy units with your veterans but kill them with your elites, to maximise opportunities for a Military Great Leader. There's what you do with the units left in a stack if you have overjudged and you win the fight that the stack is engaged in at the beginning of the turn two-thirds of the way through that turn. And so on; the more sophisticated Civ 5 is, the more options there might be here.