I did about 3 or 4 raids on the Grunt Training Facility to accumulate around 30 perk points per run. I have well over 1000 perk points, less than 1/4 of it came from attacking grunt facilities. Half came from simple A2A combat, hell I usually get between 8 and 12 perk points from just shooting down other planes. Using the 190A5 or the La5FN or the P47D-25 is good for the perkies.
I don't think reducing the number of points for attacking an undefended facility will really reduce anyone's ability to achieve enough points to buy themselves a perk ride. If the only way they can get them is through a few sorties to an undefended grunt facility in a jabo, then they, as Thunder already stated, won't be able to hold onto their perk plane for more than a single sortie.
Afterall, perk planes don't offer that much of a cushion space between their counterparts to allow a dweeb to kill an ace without losing their perk ride.
Fly them less preferred rides and your perk points sky rocket. That's the whole point behind the system anyway.
-SW