This is what I didnt get about the multi-AZ claims. If you're in multiple zones within the same region when that region goes down, you're still going down too. Yet, this seems to be the agreed upon solution decreed far & wide.
It helps. It's not a silver bullet. AZs tend to go down more often than whole regions, is the belief (I don't think there's sufficient data, but a priori it does make sense).