Crediting. As far as I know PayPal Mass Payments requires the recipient to have a PayPal account (in fact, we do use Mass Payments for customers who choose to be paid by PayPal). Crediting via ACH, on the other hand, merely requires routing and bank account numbers - which is something everyone has! The whole point, which neither PayPal nor Dwolla solve, is to use the existing financial network, not create a new means of money exchange.