Good post Vio.
An argument could be made that flying one type of plane can accelerate your learning curve by honing your aim and earning an intimate understanding of that plane's advantages and limitations.
If your goal is to achieve a high score, then yes, choosing the right tool at the right time is certainly necessary.
If your goal is to survive, learn the 190D, P51 or Yak3. Especially if you side-swap, the perks will come flooding in so you can fly Tempests and Deuces to avoid getting shot down even more easily.
If your goal if to win dogfights, mastering something versatile (F4U, 109, Spit, Ki84) is the way to go about it.
I wear my Corsairs-only badge with pride. I think I could do just fine in a Dora/Temp/Yak/Spit/LA, but that would be like admitting defeat. 
This right here would be my argument. A new pilot should stick with one plane until he has actually learned how to fly. By switching planes you make the learning curve steeper. Let a new pilot take one plane and learn the basics of flying in that plane. Once he has learned the basics of flying, then the pilot can appreciate the advantages of the different airplanes. Until then, he is just fighting the learning curve.
Many of us have been around so long we forget how daunting it can be for a rookie. I fondly remember my early days when I was doing my very best to not hit the ground.
I'm with you that it's good to understand one plane and how it maneuvers with regards to BFM and ACM in order to understand the plane better. I believe that ACM and BFM should be practiced outside of the MA in a 1v1 setting to really test your limits and ability behind the controls. It also helps you really feel the plane. Once you understand ACM and BFM, then you can utilize them with any plane and get a feel for the different abilities.
The thing is though, is that for novice pilots, they don't understand what is a good turn rate vs what isn't, or what has better E capabilities off the run way vs being at high alts with speed. The issue I could see many having is not realizing that their situation is not suitable for their plane, and therfore having a harder time getting kills or not being ganged. The other thing I would mention is that even though they practice one plane for a long time, they might still never learn to take it to its full ability because they don't 1v1 or practice BFM/ACM outside of the MA. Infact I find this to be true quite a lot. So they could be flying a P51 for years and still not really take it to its full potential and hindering themselves, when they could be having a lot more success in a plane like a ki84 with better turning capabilities and 20MM cannons for quicker kills. By sticking with only one plane, they will also not understand the ability of the planes they are fighting against from that planes perspective. This can also be a disadvantage. I find that by trying new planes that are suitable for the situation, you will have a better chance at getting kills and staying alive. By limiting yourself to one plane, you are essentially missing out on other opportunities to improve. Especially when they go to a special event and have to fly something different.
For you fellas that do only fly one plane, you have been around a while and understand when you should and shouldn't roll in a situation, and also understand the flight dynamics and defensive maneuvers better. While I do think it's proficient to understand one plane to your max potential, I also believe that it's important to maybe have 3 planes you want to learn for each situation I mentioned. One geared for base defense, one geared for furballing, and one geared to longer attack missions per the planes I mentioned.
I do appreciate yalls input though.
