All is not what it seems

[I @ M]

Does the average punter know who the AIPP is? Do the majority care?

The average punter being --- those who are looking to employ a photographer and whilst they may not know of the AIPP they are more than likely to be impressed with the logo that says the photographer is an "accredited professional photographer" in much the same way as many trades / businesses have accreditation from a body that recognises their work to be of a certain standard.
It is a reassurance to a potential customer and a valuable marketing tool.