Well, not so fast. They did put a combine on MLB Network this year. That did happen. They are going to continue to do that. So they are ahead of your suggestion, and they feel like they got good results from it. (I wrote story recently about whether baseball is becoming a "combine" sport, and whether or not that's a good thing. It's still up at StlToday.com.)
The issue for Rocker and the Mets is deeper than you suggest. It actually speaks to a bit of the reserve clause that was extinguished in baseball thanks, in part, to Curt Flood. Think about what has happened here. Rocker didn't sign with the Mets. The Mets still hold his rights even though they can no longer sign him. He cannot go to another team. They cannot trade him to another team. Baseball is limited in this regard, and that means a year is going to melt off this young man's career.
Yes, that was part of the Mets' leverage to get a deal done.
It would be a better system if, having been unable to reach a deal with him, the Mets could trade his rights to a team willing to take the medical risk (there would be) and get something in return for that draft pick and also allow Rocker to begin his pro career. That seems doable and while there's not a win-win in this regard at least both parties get to move on with their careers, whereas now the Mets move on with little harm because they get pick next year and Rocker is adrift for 12 months.