Other then the fact the Bills RB's were like 28th in PPR scoring and 29th in standard as a team I can see why everyone things this is an awesome spot.
I agree with a caveat. On the surface, it doesn't seem like the ideal spot for a running back because Allen pilfers carries and rarely checks down. But the caveat is that you could also argue that Allen will stop carrying the football so much on designed runs (especially inside the five) now that they have a competent running back in the mix. Perhaps they'll even beef up the line and start handing the ball off. I think McDermott has lamented their finesse nature the past few years. Maybe they'll get to some power running.
This is a good signing for the Bills. Harris is an underrated back, in my opinion.