ΟΤΟΗ... one may find it sensible that they started developing the sensor as an MF one... and just cropped it down to be used on an their FF camera... At least that's what they did with the CCD sensor used with the M9.... they cropped it down from an MF sensor keeping MF qualities into it (the only ones that did). 
Look, Leica is NOT Nikon or Canon or P1 or Hasselblad... they are THE CREATORS of photography and they know it... The only thing they have to do is TO respect themselves, they'll never drop to a level as to explain their decisions... they just know what they are doing much better than any other... That's why they've survived Japan some 60 years ago, they've survived China or Thailand 10-20 years ago... and that's why they bought Sinar (losing money)! They just know more of photography and photographer's needs than others... Simple as that! 
I think in this case it was the other way around; they needed a manufacturer that could make them a custom designed sensor in relatively low quantities for the M. Leica needed a sensor that could deal (as good as possible) with the steep angle of light coming out of the M lenses with special on sensor microlenses.
It would be quite something if Leica/Cmosis could come up with a '
better' sensor (think DXO mark ratings et al) than the Sony sensor that other MF camera makers chose, given the likely huge difference in R&D budgets. Of course a smaller R&D budget does not automatically mean worse IQ.
Anyway, maybe in a year the S sells like hot cakes because lots of photographers get fed up with 'the unified look' that comes out of all the Sony sensor-based cameras.
