"Is that a pardon in your pocket, or are you just happy to see me?"
If the secret "pocket pardon" is a legitimate pardon that a president can grant, just think how Trump might use the power of the pocket pardon.
All of the white collar criminals currently crawling on their bellies to him and shelling out millions for any kind of pardon would be especially beholden to Trump if he gave them a secret pocket pardon. Trump could, I assume, hang onto the pocket pardon and reveal it at any time in the future—or not. Trump could use the threat of simply tearing up the pocket pardon if a recipient did not do exactly what he told them to do. He might even use it against sketchy bankers to get them to loan him money again. Or against Republican senators and representatives to get them to do his future bidding.
And he could also sell them pocket pardons without actually doing anything to document the pardons, and when, later, the recipients are like, "Where's my pardon I paid a million dollars for?" Trump can feign ignorance (something he's really good at due to already being for-real ignorant). The recipients can't sue him for Trump swindling them out of a million dollars for a pocket pardon because then they would open themselves up to charges of bribery.
The really scary thing? I think I'm starting to think like Trump.