Who knows, maybe the Ronald Jones suspension has something to do with this.
I like Davis, but I have concerns. For one thing, we don't know how well Pollard will hold up over a full season as the lead RB, and Davis leaves us with a backup that hasn't yet proven he can carry a large part of the load if needed. Additionally, both Pollard & Davis seem to be liabilities as pass blockers, and that can kill offensive possessions.
Maybe Vaughn can help out as well, but as with all rookies, he's an unknown until he gets playing time. Plus, at his small size there has to be a limit to his pass protection abilities as well.