If you take all of the touches that a FB will get over the course of the season (runs and receptions). And then you ask, does that production (or the threat of it) outweigh the superior blocking skills of a big TE or an athletic OL or DL running the same plays (assuming that they won't get those touches or represent that threat)? I think the very few times the FB is going to see the ball means that you are better off cutting the FB, activating Williams at HB, and getting Murray some rest along the way. In other words if you are keeping a FB around for short yardage situations, there are other ways to put a blocking scheme together (including TE's, and OL or DL linemen) that wouldn't require the FB roster spot.