Personally I find that re-testing thing very intimidating. We have heard stories in this forum from people that had been re-tested.
In my opinion, if you simply want to renew your medical ( let's say your an instructor for example) and the AME asks you to do the test again ,simply leave the room and try going to another AME.
BUT, let's say that you wan't to fly for a major airline, like Lufthansa( just an example). They have their own medical facilities and their own doctors and it's almost certain that they'll ask you to take the test again...
I certainly wouldn't wan to go through the same stress again and probably fail. I think it's to much.