Guy's the ability to pass a 1st class medical is not the issue. Anyone who has a current DOD flying physical will be able to pass a 1st class FAA medical, as long as they do not have strange wavers.

The reason to get the 1st class medical and keep it current is to set a track record with the FAA and build a relationship with an AME. It may never happen, but if you are the guy who develops some kind of problem which will require a FAA waiver one month after you separate, it is good to have these records and relationships.