I believe that the exchanges already accept bank wires. So wiring funds to a payment processor would be just silly.
The only advantage I see is that Dwolla and Paxum can accept ACH. Why can't an exchange set this up directly?
American banks tend to make it difficult to directly send ACH from online banking. Simply Dwolla is easier to use. ACH is both a push and pull system, so Dwolla pulls from accounts at other banks and then provides a simpler interface than the banks do for sending those funds out again.
So why couldn't an exchange or Bitcoin vendor do the same as Dwolla? Just make a convenient interface to withdraw directly. It can't be just about the interface.