Keep in mind that if you truly weigh out and balance the ENY / Perk system for every plane in the set, you'll be putting newer players (and older but still less-skilled players) at an extremely unfair disadvantage.
Setting the system up "perfectly" would mean that only the players that had been here long enough (and had scored enough kills/damage) could afford to buy the "best" planes. Even once the cost of a plane has been "earned" the expense to a newer/less-skilled player is much higher than it is for an old "ace".
The cost to both players my be 20 perks (or whatever) but it's a lot more difficult for a lesser-skilled player to come up with those 20 perks. In addition, the lesser skilled pilot is MUCH more likely to burn/waste any perks they put towards a "better" plane than the guy with enough experience and skill to bring that ride back to base safely... As an example, my favorite ride in the game is the F4U-4. I have enough perks to buy them anytime I want to, and really need to screw up to lose one. And if I do lose one it's no big deal, because I have GOBS of fighter perks. Losing a perk plane is no big deal to me. The more-likely result of me buying that F4U-4 though, is that I'll return it to base, and actually put ADDITIONAL perks into my bank.... Compare that to a new player who's just gotten enough perks to buy that F4U-4 (and gotten slaughtered relentlessly in the process). He/She lifts that shiny new plane, heads into combat, and loses it to a guy in an A6M2 or SpitII.
In a difficult game where new players basically amount to being "sacrificial lambs" for a LONG time, only allowing them to fly the cheapest, lowest performing "free" planes while all of the skilled players are able to purchase anything they want wouldn't be "fair" in the slightest. In addition, it would be just plain bad for business...
So in the end, there need to be several different "free" planes, in several different "flavors", that can compete with the best in the game; and in addition, these free planes must be ENY-rated high enough to allow those lesser-skilled pilots to earn some "easy" perks so that they can begin to fly some of the perked planes that they're striving for.
The P51, LA-7, Spit16, etc fit that mold perfectly.