I am an SP player, so naturally A.I. is very important for me. I also think that an MP campaign would be impractical due to the amount of time each turn would take. Also, since it would be hard to fill up all factions in a MP game (everyone wants to play the English, no one the Aztecs), the an A.I. is necessary to make up for that.

If it was possible to make the strategic part simultenously turn-based (e.g. the players order their moves, press end turn and then these moves take place, like it was in S:TW and M:TW) then it might be workable. Such a game might entice me to go MP as well.