Beane's overriding impetus seems to be flexibility. Every one of his moves, signings, the contracts, etc seems to be based of making sure he isn't forced to make a move. They will almost certainly draft a WR, but Higgins bombing his workout would have reduced the flexibility at 22 into taking someone they may not believe to be a '22' player. Diggs in the fold means they can virtually go any direction they want on draft day.
..and I have a hunch with the run on WR and O-line that seems to be coming that the Bills draft a defensive player