I think its perfectly reasonable to remove the Tank-Healer-DPS but personally i'm a pretty big fan of it. I think the healers and dps can remain unaffected. What ruins the whole AI is that the tanks have Taunts and other abilities that artificially add agro for nothing. Please, someone tell me why the boss should be attacking the tank, its not logically at all. The tanks should be trying to protect healers and create barriers and slow down bosses so people can run away. Their job is to protect healers and DPS but not necessarily by taking damage even though that could come into play a little.
An intercept ability is a good example, the boss attacks healer 1 and the tank realizes they're gunning for him so they attempt to intercept the damage by placing themselves between the boss and healers.
Like the above examples, spells in the current state of the game are based on targets, but why should a spell go through 2 people and hit someone behind? Can't a tank use his shield to reflect or absorb the damage if he is correctly positioned in front of the DPS or healer that the boss has targeted?
Something I think you focused on a lot Dave was formation and the bosses working as a team. This is a good idea as well, part of the problem with current raids is that we have 10-25 people vs 1. The boss if all alone needs to be extremely powerful to even stand a fighting chance. In essence this is what causes the tank roll, nobody can take these huge hits but the tank. Without the huge hits the boss would never be able to kill anyone. Lets say that we had a fight 10 players vs 10 npc where the NPCs were working together just like the players. WoW tried this in WotLK I thought it worked out pretty well thought it could have been improved since the NPCs were not working together and the taunts still worked (at least temporarily).
Anyways I think that these responses answer my original question, its not a matter of computers not being able to perform this kind of AI real time in a game, its a matter of a game design that doesn't allow for this kind of AI in its current state.
Most of the current ways things are done in MMOs are to overcome obstacles involving latency. Your ideas on positioning (intercepting attackers, physically blocking attacks, etc...) are of course good ideas, but they tend to fall apart in a simulation where oft-times large latency is present, which is pretty much every MMO. Everyone who has played with huge ping times is familiar with rubber-banding and other occurrences, and these issues are only aggravated by the requirement of precise positioning. A raid wiping because they failed the script is one thing, but a raid wiping due to latency issues on the tank's part is a whole different ball of wax, one that annoys the piss out of the players when it happens. With tight timing and positional requirements plus latency, this type of wiping would likely become the rule rather than the occasional occurrence.
Even with low latency(Which is achievable with nation or region restricted servers for example) that kind of gameplay would be problematic at the MMO scale due to the increased bandwidth requirements (Less complex gameplay lets you squeeze more players in, (even if you can afford more bandwidth on the servers you will restrict your playerbase if the clientside bandwidth requirements get too high). (You can have tight positioning in a MMO if you want but you have to sacrifice something to get it)