I haven’t heard anything more official, but this is a smart theory proposed by both Carter Allen and Lou Miranda:
Battery life is critical on a mobile device, and it’s clear that Apple puts a huge premium on battery-saving technologies and techniques. Processing a native binary format is much more battery-efficient than parsing, re-parsing, generating, and re-generating XML or some other human-readable format on the fly.
This is the likeliest theory I’ve heard. It’s disappointing that it isn’t human-readable, but it’s also disappointing that it’s undocumented. I’m doubt there’s going to be a mad rush to ensure that third-party apps can read it in the way that they can read Word documents, too, which is also disappointing.
But if you use Pages on your iOS device and Pages on your Mac, as I do, you’ll be happy with the ostensibly-improved sync.
Update: Drew McCormack has a similar theory:
They have split that potentially large XML document into many small binary files. Each file can now be loaded in isolation, and this is much better for iOS. Effectively, they have built a partial-loading document format.
Makes sense to me. The part of me that hopes for inter-app compatibility and future-proof formats is a bit bummed, though.