Well maybe have the medal requirements non-linear - first medal is easy to get, second is harder, 3rd is quite a bit harder than the 2nd, etc.
As you say after you've been here a while you realize earning medals is pointless, but it may serve as the extra incentive for some new guys to stay here long enough to reach that point?
In fact it in small way, it might be considered something the new AH player has in common with WWII airmen - just as the real airmen came to realize medals were meaningless, so too, would the newby-turned-vet AH player. If he stays long enough to become a vet, that is...but in the meantime maybe while chasing that goal, he's brushed off the pounding he's taken and stayed motivated long enough to become competent?
This might have some application in scenarios, too. Medal requirements could be different, but pilots and air groups could earn distinguished service awards, etc. Could build esprit-de-corps and and add the sense of immersion that Fencer and company are trying to encourage in scenario play.