...but in a single day datacenter, which realistically drops several "9"s from that guarantee when you consider tail events.

I like b2 and am using it for backup in some projects, but unfortunately it's still not an appropriate choice as a primary data store.

The common counterargument I've heard is that you should have multiple redundant storage providers that you manage at the application layer anyway. Problem with that is, there are a near-infinite number of things I "should" do, but very finite resources to do them. S3 backs up across multiple availability zones by default, so using it means one less thing I have to spend brain cycles worrying about.