Almost none of the aircraft in AH are employed in accordance with their historical missions on a day to day basis in the main arena. P-51s would mainly be found over Europe at altitudes above 20,000 feet escorting massive formations of B-17s and/or B-24s followed by strafing runs on targets of opportunity... with long periods of doing nothing flying back and forth between England and Germany. It rarely happens in the MA and maybe only a little more frequently in special events.
The P-61 without a radar would be just as fun to fly as a Mosquito, P-38, Bf110, Me410 or any other twin fighter. While the aircraft are modeled reasonably well, AH is far more a game than a sim. The point is to have fun flying various aircraft that you happen to like. I like the P-61 a lot more than many other aircraft in the plane set. From many posts over the many years, I would say a significant number of other people feel the same way. The fact that someone has flown an A-20 solely as an air superiority fighter and has a kill record to be feared confirms that the P-61 could find as good a niche as any other famous/popular plane regardless of whether it is employed per historical mission or not.
AH is a game with a great plane set. But the plane set could be larger and there is nothing but time, money, and the arbitrary whims of HTC keeping the P-61 from becoming a member of that plane set. The fact that Ta152s, Me163s, and Me262s are available defies any arguments about production numbers, time in service, effectiveness, significance, etc. It is simply a matter of HTC deciding where to spend their limited resources. It took a long time for the P-40 to get added. Ultimately, it was one of the aircraft that got upgraded and even improved to the AH2 standards. The new F4F/FM2 is great! Bringing all the old aircraft up to date is a huge chore and was never finished during the time of AH2. But adding new aircraft helps add interest. New models/textures don't really change game play nearly as much as the chance to fly new aircraft. Unfortunately, modern graphics standards have greatly increased the time/effort/money it takes to do a single aircraft type. So AH doesn't upgrade/add aircraft nearly as fast as they once did. New aircraft, whether an He111 or a Brewster Buffalo are always welcome. I would welcome any new aircraft, but would greatly prefer the ones I liked so much that they are hanging from my ceiling after hours of gluing/painting/decaling. The ones I have previously mentioned are the ones I would like to see in no particular order: P-61, PBY, SB2C.
No amount of arguing will change my preferences. I like what I like and ask for what I like. HTC has and will continue doing whatever it wants. Sometimes, what they want and what I want overlap quite a bit as evidences by the overlap between my model collection and the AH plane set. I would be happy to see the overlap eventually reach 100%.