One of the problems is TT, for example I have 10k in fighter perks, 7k in GV perks and 1.5k in Bomber perks.
Its far easier to get fighter perks (in my opinion) vs bomber or GV perks based on overall time per hour (meaning in 1 hour I can get triple the fighter perks vs buff/gv's)
It would be off balance simply because fighter jocks would easily rack up the perks, while a newer player - would struggle getting them, off balancing the system since one could farm for an hour and then up a King Tiger while a newer player gets stuck still trying to get perks.
Its not so much for someone whose a veteran player, opposed to someone new.
Then again I like the idea of interchanging the perks, or even to squadies (I brought this up very long time ago) to allow certain amount of perks to be transfered to another player while they take a hit for transferring (Lets say 200 perks max you lose 50 perks for the transfer).
This allows a veteran player to transfer some perks to a newer player, while he's able to replenish those perks quicker.