The problem of using multi threads with AI is that it is very hard to avoid race conditions.
I will use pathfinding as an example because it is the most common use of AI I have seen in this forum. Say you are running A* on your graph, if one thread has processed a node and another thread changes that same node, the result won't be reliable. Worst results may happen, as referencing a node that no longers exist, resulting in a segmentation fault.
Using a task system is a good way to have your system executing several independent parts of the code at once (for instance, physics simulation, rendering, sound and particle effects math), but when it comes to IA is not that simple. Of course that this depends a lot on how your system works, if, for instance, your game doesn't allow path blocking (common on tower defense games), you may run pathfinding algorithms at the same time.
So, this is the best way to use threads on a game, but hardly likely to be the one for AI, also you must always keep in mind that you must no introduce race conditions.