It is not a benefit if that money isn't actually there. They provide no guarantee that the funds are yours to use since they can be hit with chargebacks and will just take the money from you. This is just as bad as paypal.
Doesn't amazon payments have the same problem?
I don't know anything about amazon payments, but anything that is dealing with the US banking system via ACH or the credit card system is exposed to chargebacks.
Dwolla is a benefit to merchants. Under $10 no fee over $10 .25 (see how long this lasts of course). Presumably the merchant because they are saving money will offer an incentive to the buyer to make the trouble worth it, at least in theory.