Plus is only one of many interbank networks, though (I think) the largest and one of the few nationwide ones. Those networks all interoperate, though. They function as the intermediary for both ATM and PoS purchases for ATM/debit/check cards.

It seems like they would be the best candidate for direct transfer of funds between people. I don't know why something hasn't already come up. I would guess that either there's some legislation involved or they don't think it would be profitable, or both.