I completely and utterly disagree with the "It's the pilot, not the plane" theory. It is crap, doesn't hold up to any real scrutiny. Does the pilot "matter"? To a certain degree, yes. I'd say the ratio is probably 80/20 plane/pilot.
Put two pilots that know the relative strengths and weaknesses in two different planes, it becomes 90/10. Put a crappy pilot in a good plane and it becomes "How badly can I screw up before my plane can't pull me out of it?".
"Dweeb planes" are the planes that do the work for the pilot. The label "dweeb plane" is contextual, in that in some situations a plane will do all the work for a pilot, and in some other situations it doesn't have the performance vis a vis the enemy plane to save the pilot when he screws up.
Another definition, and one that I favor, is the "dweeb plane" is one that makes all the other planes relatively pointless to fly. Why fly a 109F, when a Spit 9 (or Nik) does everything better? More firepower, faster, climbs better, turns better, etc. Why fly the 109F at all? Quite a few of the planes that are "rare" are completely overshadowed by the Spit 9/ Nik.
All of the LW planes, with the exception of the 109G10 and 190D9, suffer from the fact that while they are no faster than the Spit 9/ Nik, they also have less firepower and are much worse in a fight. The Ki-61/F4F/F6F/P47/P40/F4U (with the exception of the C-hog, which has a firepower advantage) suffer from the same thing. The P-38 has found a niche as a suicide bomber (praise Allah), if it didn't have a payload as large as it does, it would go relatively unused. While the A6MX has better turn performance than the Spit and Nik, it is much more frail and lacks the "one shot kill" firepower, so it goes relatively unused. C202 doesn't have the firepower to be competitive, nor does the 205, and even if they did they are still out-performed by the Spit/Nik, so they go relatively unused.
The sole exception to this rule is the spit 5. I think many people that had been in the Spit 9/Nik found it to hard to kill someone from a "nuetral" start that was also in a Spit 9/Nik, so they decided to fly the Spit 5. While the Spit 5 is slower and doesn't climb as well, it turns better, and more importantly gives up nothing in the firepower department. This means that once they sucker a Spit 9 or Niki into a turnfight, they can rack the stick back into their stomach and still have the one shot kill capability that is so important in the MA.
So that leaves us with the planes that are faster than the Nik/Spit 9. The 190D9 and 109G10 are basically gimped versions of the La7, which would explain why they are about 1/3rd as popular. The Typhoon has found favor amoung die-hard cherrypickers (with 4 Hizookas, it has the most firepower possible in AH), and has a strong following amoung building battlers that really don't want to fight people. The P-51 camp is slowly losing followers to the La7, since the La7 is superior in every area of performance. It will never really become "rare" because of its historical appeal to so many people, but the diehard "min/max" crowd will stick with the La7 because it offers performance superior to every plane faster than the Spit9/nik, combined with the speed to escape from any situation the pilot can get into.
Man, my post isn't quite as long as Kweassa's, but it is getting up there. Perhaps the best illustration I can give is an experience I had in the Ma a while ago. I was flying around, got into a great fight with a guy in an La-7. I was in a 109F-4, we went round and round in a very well flown, stall-speed, knife fight. I still believe he probably would have won, except he hit a mountain that I managed to creep over at about 95 mph. I toss a for a good fight, he tosses one back.
Later, I run into a Spitfire and a 109, both higher than me. Spitfire was a Spit 5, 109 was just bore n zooming so I never got a decent look at him. 109 dings me as I'm fighting the Spit, Spit kills me. Lo and behold, it is the same guy I just fought in the La7. He tosses out a , I ignore it. He PM's me with, "what, no when I wax you?". I replied "no, I only after a good fight". The guy started flying a plane that had superior performance to mine, and lost. So he comes back in a plane with even better performance, and with a friend. I imagine the guy hasn't seen the inside of any plane other than a La-7/Spit since he started playing. He picks what he will fly based purely on what kind of advantages he needs to make sure he won't die.
I don't have any respect for people that do that, and since the "Big 4" (or 5, depending on who you ask) offers such a gigantic performance advantage over the rest of the plane-set, it is safe to assume that the vast majority of people who fly those planes fly them purely out of a desire to have the biggest performance "edge" they can possibly get. Thus, the "dweeb pilot", makes the "dweeb plane".