The main reson most don't want an AI to take a humans place (when they quit/DC) is that it "Feeds" the oppoents. What if replacement AIs gave only 50% (or 25%, or even none) XP/Gold to the team that kill them?
You'd want an AI then (rather than nothing); worst it could do is keep the enemy busy for a bit; attack towers; kill mobs; make a good meat-sheild; main thing is, it could'nt "hurt". AI replacements now "hurt" you, and they are suppose to help fill the void some, not spell the end of the game.