Draw out power, fine, but you never saw Ishida trying anything like that . What I'm saying is no matter if the Quincies can draw out power from the bankai thats fine. But they should not be able to control that Bankai to allow the power to be released in the first place. If in a fight with a shinigami, I'd accept that they might be able to draw from a bankai because its power was being released by its rightful owner. But to claim that they are able to release the Bankai itself is just nuts.
Sure a Bankai is a source of power, but that power comes from the Individual shinigami upon creating his sword and pouring his power into it over a period of time. So how could bach "release" that power if it isn't his. In the Kenpachi vs Ichigo match Ichigo did ask Zangetsu for help. He didn't just combine powers. So in much the same way, how will Bach convince the "Sword spirits" to release their power for him when that would mean betraying their previous owners.
Lets consider two possibilities.One, a "sword spirit" gains its power itself or two it gains it from the shinigami that uses it. Now, if its from the shinigami, it creates the problem I mention above. Some bankai's are naturally more powerful that others. If the power comes from itself, then we need to ask ourselves, what makes one "spirit" stronger than the other. It comes down to the Shinigami(again) as the Asauchi do seem to be of a standard quality.