AA exists for very different reasons when compared to other providers.
Which I think is fair enough personally. They have a very different model to Zen who are more bandwidth, multi-vendors, lets throw as much kit at the problem as possible.
It stands to reason that AAISP sticking steadfastedly to a single vendor, makes their job of maintaining a high quality connection much much easier. Also allowing their customers much closer access to their monitoring, etc.
Almost all ISPs are quantity over quality, AAISP are invaluable to people like Weaver who needs to keep things working at all costs.