As has already been said, RBs are (1) disposable (ie, short lifespan in the NFL) and (2) useless behind a bad O-line. Hell, it took Barry Sanders absurd skill to even put up decent numbers in Detroit. Spiller is no Sanders.
The Bills need a long term investment (OT preferably, DE/NT less preferably, or even QB). RB is not a position of need and RBs are completely fungible. Not many teams ever sucked solely because of a bad RB. Lots of teams suck because of a bad QB or terrible line.