Mountain Lion will be the first release in a new annual schedule for OS X. Like many of Apple's moves it's initially surprising, but hindsight makes it seem obvious. I'm sure there are several benefits to an annual release, but I would propose that iCloud may be the largest.

We know from Tim Cook's comments that he views iCloud as a "strategy for the next decade or more". Rolling out an iCloud service doesn't just require server code and a giant data center, it also means client-side code in every Apple computing device. And it's not just an app: given how iCloud services are tightly integrated into the operating systems, an iCloud service also means an update to iOS and OS X.

More like Documents Only in the Cloud

We have an example of an iCloud service that was launched without coordinated support from iOS and OS X, and it was ugly. Documents in the Cloud launched only with iOS support. Since Apple hates to describe features they don't support, they didn't volunteer that there was no support in OS X. Documents would sync between iPads and iPhones, but not OS X computers. At best you could manually access the document via Apple's web site. It was a glaring omission, and made Documents in the Cloud nearly useless for a lot of daily tasks.

This lack of integration baffled reviewers and users alike. Apple usually shoots to delight customers, not to baffle them. It didn't "just work". Worse, it will have been close to 9 months before OS X gets Documents in the Cloud capability.

iCloud Requires Coordination

A yearly OS X release schedule allows Apple to coordinate iCloud service releases with operating system releases. Any framework updates to support new iCloud functionality can be baked into iOS and OS X and launched together in coordination with the service. The closer the releases are, the more seamless the iCloud service will appear. And if it's not seamless, Apple won't be able to win back customer trust from the last several years of janky network services.