You could look at it this way. Perks could be used to fix minor damage, but realistically. Things like fuel leaks can be patched, and guns can be replaced, so you could do things like that. Another thing is time. Fuel leaks aren't hard to patch up, but you'd need to go to the ammo dump and retreive one or two or even three new guns, so that would be more time consuming (not much...gameplay). Wings, engines, tails, and props would require new planes, as those were major overhauls to fix, and usually the plane was scrapped anyways...