From the HTC webpage:
"The perk system is a way for HTC to introduce some interesting but otherwise unbalancing planes on a limited basis but the benefits go deeper than that. Perk planes (and vehicles) would be things like Me 262s, Ta 152s, Tempests, B-29s, Ar 234s, Tiger IIs, etc. These are interesting rides but would be very unbalancing if they were available on an unlimited basis. So there won't be unlimited availability but they'll be available as bonuses or perks every so often. "