I am surprised this thread has got to 18 pages.
This idea is just terrible, in theory and in practice.
In theory, because it would be an absurd amount of work to coordinate an AI that is notorious for being lacking, your own character, the boss' movements, and all the different tactics involved. Some bosses are already hard to nail if you are your own self without the nervous breakdown provided by ranged tanking.
In practice, because the AI will either be utter crap, and so you'd just pick a conventional tank, or phenomenal on its own, so you become mandatory in any content.