Mike Maccagnan explained the Jets' decision to trade up for Brandon Shell.
I am not really sure I understand the compensatory pick angle Maccagnan uses. If his projection is correct, the Jets would be getting that extra pick whether or not they made this trade. I can understand it playing a role in a decision to trade for a veteran player. In that case, you're getting a proven commodity while keeping your team with a full assortment of picks. I could even see it doing a different type of trade up. In this case, the team gave up a lottery ticket straight up for a less valuable lottery ticket.
That is not to say this will necessarily prove to be a bad move. It all depends on how Shell turns out. On average, you lose more than you win making deals like that, and the odds of hitting in the fifth round are very low. I hope this is an exception where Maccagnan saw an exceptional talent available and made a move. This is not the type of deal a team should make a regular occurrence, only in truly unique cases.
I can only guess that the Jets had a third round or higher grade on Shell. Then it makes some sense to give up a fourth round pick. You could conceivably get something of greater value. If the Jets had a fourth round grade, it would make less sense. You have to figure fourth round graded players will be available to you next year in the fourth round.