Apple's real message is that if it can get all customers on the latest software — a model that has worked for iOS since two-thirds of users have iOS 7 now — the company gets more customer loyalty. In addition, hardware will seem old with the latest operating system for laggards. As a result, it's possible that handing out the latest OS X for free will mean more MacBook upgrades.
A few implications on the Mavericks freebie move:
Apple gets its latest software and technology in the field
Mavericks is likely to be tied into Apple TV better. Apple TV is a precursor to an eventual iTV
Hardware upgrades are likely to follow
Apple is betting that the OS sales are a thing of the past. The real money revolves around services like app sales, subscriptions, e-books and iTunes Radio.