From what I saw this season, KM's inactivity was a function of the numbers game. Knowshon and McGahee were playing too similarly in this offense to keep both of them active. Obviously, Willis has the edge in talent and production, but I wouldn't say it was a huge disparity. If Knowshon had been playing ST, I really think that he would have been active over Ball.
But Ball does play ST, he's on Punt and Kickoff. As a result he was active because he brought more value. With McGahee out, I fully expect KM to be the next man up and to do a damn good job at it.
My post is full of "I thinks", so there's not really a huge amount of objective information to back this up, but I look forward to the rubber meeting the road this weekend against the Chiefs.