There were a few years where BB was picking up a bunch of players that washed out of Oakland. And there was a phase more recently where he was picking up players that washed out at Tampa Bay. This year, it seems like he's picking up players who flopped in Chicago.
To me, buying low on multiple players from a specific program is a "tell" from BB the GM, that he thinks that roster has usable talent that is being mis-coached... he thinks those players, in a better and more productive teaching environment, have unrealized upside to cash in. It's the basis for making a value-based purchase. Ever the economist.
BB has told the media, in the past, that what a player has done in one system isn't always indicative of what they would be in the Pats system. As mentioned, Vrabel was a little-used depth player in Pittsburgh before BB made him a star. Welker was an little-known depth guy at Miami until BB played to his strengths. Plenty of examples. Rodney Harrison was washed out of San Diego, and Randy Moss has lost a step in Oakland. Etc.
Picking up all these cast-off players from the Bears makes sense to me from the perspective that Chicago has been recruiting players of a profile that fit the Pats criteria-- e.g. we all scouted McClellin for the Pats coming out of college, and he was general consensus a higher potential prospect than Chandler Jones that year. The Bears franchise has been struggling for years, but we don't have to assume it's because their players don't have talent.
Cooper, Hogan, McClennin, Bostic... these are all "buy low" players who don't have a big market for their services. We know BB loves panning for gold, it's how he finds Edelman, Lewis, Ninkovich, Stephen Neal, Malcom Butler. He shops in the discount aisle and buys a lot of trash, mounds of it. Then, he relies on one of the strongest coaching staffs in the NFL to polish them up into usable players.
I promise that a few of these buy low contracts will flop, but some will hit big. It has been the philosophy of this regime ever since BB got to Foxboro. Whether McClellin hits or not, be assured that BB thinks more likely than not he got a good deal on him.