here are 3 reasons why:
1) To give "hanger queens" more of a chance. When's the last time you've seen a C.202 or a KI 61 flying around? In my opinion all 4 engine heavy bombers should be perked, as well as all late war fighters (i.e. P-51D, Spit 16, ect.....). also in the case of aicraft that have multiple variants (i.e. BF-109, Spitfire, ect....) the earlist one would be perk free while the next model would be slightly more expensive.
2) encouraging more variety of aircraft. The late war arena is filled with P-51D's, BF-109G-14's, Spit 16's, P-47D-11's, and B-17's. I don't think it would hurt anyone to try a new aircraft
3) Make more people want to land their plane. It would reduce the number of "Kamikaze" F4U raids off CV's and encourage people to take more realistic fuel loads.