AI can be ranked as long as the AI can also lose rating.  The way the system works is such that if the AI truely does suck a lot it's rating will go down.  As it's rating goes down it becomes worth less and less to real players when they beat it.  Eventually the AI will get to the point where a high ranked player will only get a fraction of a rating from the AI but they will lose a ton if the AI wins.

Assuming there are actually real players involved as well, the AI matches will basically become fillers for high ranked players and low ranked players will always be able to find a game that is challenging (against the AI).

Of course, ideally the system wouldn't match a low ranked player (the AI) against a high ranked player due to the large difference in rating.

The AI cannot be involved in anything that is to be taken at all seriously in a competitive way.

The concern is not current stats, but the trend they point to - namely that a player can grind Pantheon AI for easy wins which raise the player's experience/rank.  Grinding against bots shouldn't affect ladder rank, or if it does it should work the way Micah describes it above. 

Bara has described experience as follows:

Right now the bots seem to be delivering the experience.  Does that change when the player pool is larger?