I am not saing that the 51 is used to run. However, in an environment such as the MA, with red guys at 1k, 5k, 10k . . .20k some turners some speed deamons, most planes without at list one wing man, are death traps. There is no way you will survive the MA in a F4U for example unless there is team work involved. So, having a plane like 51D that can run or turnfight, can help you get out of troble and chose your batles a little better when you are by yourself. After all, no one wants to be just a target.
Yes, you could beat a spit or nik in a p40 for example, but after you kill them, you pretty much have 30 seconds left before his friends blow you out of the sky.