Not just by BK; most pickup manufacturers use a variety of different magnets in their humbuckers, but I've never seen one with an A3 magnet.
I am assuming there is a good reason, and not just 'vintage PAFs didn't use it, so we won't either.' Some people, maybe including Tim, must have experimented with this combo and found a good reason why it was inferior to other magnet types in a humbucker, even if it works well in a single coil, so what is it? Too weak or thin?
Edit: While I'm asking stupid questions, maybe I should also ask why you never see Alnico IV single coils?