In any case, apparently there are a number of people (including, perhaps, David Allen) who find that AB priority codes or equivalent work for them, but not ABC priority codes. That's fine; but there are other people (including myself) who find that a larger number of gradations in recorded priorities of next actions is more effective for us.
Maybe some people hate the letter "C"? In this context I hate "A" and "B" too... I don't need any priority gradation except for Active and Someday/Maybe.