If you are comparing a past cruiser Military discount price versus a new cruiser military discount price, then I agree you will not see much difference. The mitlitary discount trumps anything a past cruiser normally gets. (My PVP always asks if I will be getting the military discount after my mother cruised with us and she got the whole cabin my father's discount.)
But for a non senior citizen/ non military fare, yes I get better deals being a past cruiser. Most would be from the advanced notice we get for fare sales (a P.C. normally gets a day early notice and booking for fare sales.) These allow you to get a cabin that may be sold out before the sale goes to the public. The biggest thing is to be able to go "whenever" and be able to take advantage of the off peak cruise dates as where thats where the best deals can be found.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007