One of the most surprising things to me when moving to NYC was how often I would NEED CASH. It seems, anecdotally, that most restaurants and bars in the village are CASH ONLY (also most cabs prefer you pay in cash, and can be real dicks if you try to pay with a card even though legally required to accept them)
Personally I would rather not carry cash for security reasons, as well as convenience reasons and it seems like the rest of the world is headed in that direction (I remember when it was a big deal that McDonald's started accepting credit cards).
NYC has always seemed a little techno-phobic to me (also obsessed with OLD things: antiques, old apartments, etc.) and I think this explains why many businesses here don't accept mobile (or even credit card) payments just as much as the additional overhead cost (not to mention that you can better hide cash transactions from the tax man)
But now I'm just embarrassed that Africa seems to have better payment and p2p transfer technology than NYC in the country that invented the mobile phone.