I think I have the answer. McIntyre was a RFA during the last FA period and the Bills brought him in, but he probably wanted more money than the Bills were willing to offer (a la Bryant Johnson), so he wasn't given an offer sheet. Getting no better offers, he re-signed with the Falcons for one year (again a la Bryant Johnson), only to be cut before the season started. The Bills probably still liked him, but since it was so close to the start of the season and Barnes knew the system, they decided to see what Barnes had. After failing to impress, the Bills made the move. Barnes is healthy, or at least healthy-enough to play, and even if he weren't, I don't think he could file a grievance since his salary for the season was guaranteed anyway.
From the sounds of it, McIntyre should be an upgrade in the run-blocking department, and especially on ST's. He'll probably be no worse as a runner and receiver. That is, once he learns the system.