I don't think AI is a good argument against stacking; 1UPT is much harder to manage than stacks. The Civ IV AI did a much better job managing stacks than the Civ V AI does with 1UPT.
Yes but at the expense of making combat very tedious for the Civ4 human player to play. While the AI has a harder time with 1upt (as the AI does in every computerized traditional war game), it is still not easy at higher levels (try spawning next to Greece or Aztecs). Plus the AI compensates for it in other ways, which makes the end results a much more fun game to play with some challenges at higher levels.
Wow, I can't believe so many people who hate 1UPT are playing Civ5!
I wouldn't assume they are even playing the game.