Phillip Adams allowed 456 yards in 175 cover snaps in 2014. According to Pro Football Focus, there were 116 cornerbacks in the NFL to participate in at least one quarter of their team's cover snaps. No corner in football allowed more receiving yards per play in coverage than the 2.61 Adams surrendered. (For good measure, Darrin Walls' 2.00 was the fourth worst.)
Adams is a journeyman. He is 26 and will be 27 before the next season starts. The Jets were the fifth NFL organization he has played with. He is a former Seahawk. John Idzik's familiarity with him might have led to his signing.
If anything, familiarity with Adams probably should have led Idzik to a different direction. While there are seldom good fill in options to sign during the season, Adams played the cornerback position as ineffectively as anybody you will ever see. He had one shining moment, a nice interception of Philip Rivers in San Diego. The rest was not particularly pretty. The first time he stood out was allowing a big touchdown in the second half against the Bears. He inexplicably abandoned his zone leaving Martellus Bennett wide open for a touchdown.
I haven't seen many cornerbacks with as little of a feel for the game as Adams. The cushions he provided were usually way too big. He frequently took himself so far out of position that he couldn't make plays on balls thrown down the field or use angles to stifle receivers making cuts. Darrelle Revis is great because he's a tremendous athlete, but he's also great because he anticipates the route his man is running and positions himself to stifle the receiver at the point designed to help create separation. The way Adams played frequently allowed receivers to do whatever they wanted.
Adams is an unrestricted free agent. I think the Jets should probably let him go elsewhere. He didn't show a whole lot of upside, and four other teams have already concluded the same thing.