The guy came into the league as a 5th round pick because he has a weak arm, inability to recognize the blitz, and inconsistent accuracy. He had never had to carry a college offense on his own and had played with a lead with almost every single snap he took in college. He was compared to Matt Cassel as a best-case scenario. Since he's entered the league, he's done very little to shake those comparisons.
When Dalton got hurt in 2015, McCarron was efficient but unspectacular in 5 games, but the Bengals' offense got slowed to a crawl compared to where it was with Dalton. He couldn't put them over the top against a frankly inferior Steelers team in the playoffs. He's done nothing since.
I don't see what he provides as a backup over Brian Hoyer, so unless the latter is planning to retire this doesn't make any sense to me. McCarron's the latest in the series of later-round quarterbacks who end up hyped to the moon without any sort of real world evidence that says they're any better than they were when teams passed on them 4 or 5 times several years ago. This isn't Jimmy Garoppolo sitting behind Brady and impressing every time he saw the field; this is a guy sitting behind Andy Dalton and looking fine at best the only chance he got.
Frankly, the only way this makes sense, and those words are doing a lot of work, is if they're trying to deny him to the Bills as a caretaker for whatever rookie they end up with (or trying to drive up the price), but it's going to be a stupidly expensive gambit if it works. Would you rather have Dion Lewis or AJ McCarron?