John Conner's second tour of duty with the Jets came because Tommy Bohanon suffered a season-ending injury early in 2014. Conner was a 2010 Jets fifth round pick who was cut in 2012. After stints with the Bengals and Giants, the former Hard Knocks star returned to his original team for 12 games.
Conner's impact was limited. A traditional blocking fullback, Conner only played 154 snaps, 92 of which were run blocking snaps. He only touched the ball 8 times on offense. He just isn't the type who should elicit strong reactions at this point. He is a lead blocker. He is neither the best nor the worst at what he does.
The fullback position is being phased out of the NFL, but I still see usefulness in carrying one. It has utility when a team is trying to play power football. Since it is being phased out, fullbacks tend to be cheap. Having a player with situational use is probably more useful than the final roster spot would be utilized otherwise.
I don't have a problem with the Jets bringing back Conner to compete with Bohanon for a roster spot in training camp. I also don't have a problem if they don't, assuming they find a prospect they like or somebody more proven.