You make some valid points.
At the risk of sounding silly, I believe that MM is willing to do whatever takes to get the best personnel on the field at any time.
That said, I'm thinking that WM just isn't "there" yet. Having had knee surgery before, and granted, I wasn't in the conditioning that WM when he was hurt, but it still took me a good 3 years to be comfortable enough and trust it enough to play in a no-check hockey league.
An NFL RB is probably more prone to having his knee twisting and stressing not just running, but in the tackles and pile-ups, especially if said player has a tender knee, than I ever would in a house B-league.
Barring that, I don't know why we haven't seen more of Willis...