Quoting the pixel size down to which your lens performs is a useful short hand (as opposed to contrast vs spatial frequency) way for telling the guy in the street, 'you know what, the detail that this lens can resolve is a worthy match for your digital back.' I'd rather be out shooting pictures than pouring over MTF charts - I just need to know that for $9k this lens is supposed to sing, not just hum out of tune.
To quote Schneider-Kreuznach: "The pixel size and pitch of a CCD array would clearly determine the necessary performance of a lens for use with this array".
Well, they know the pixel size and pitch of P1's flagship product - the IQ180/280 - let's see if this has determined the necessary performance. My prediction: The lens is capable of producing great images, but doesn't exploit a 5.2 micron back to its fullest.
If we have to play the MTF game, all P1 need to do is publish some graphs, and I'll shut up.