It's not about user-facing features. It's about the API and OS bug fixes. I upgraded my iPhone 4S from iOS 5 to iOS 7 not because I wanted iOS 7, but because I was unable to download new apps. Everyone in the App Store was already targeting exclusively iOS 6 and I didn't want to get further left behind. In the iOS world, developers target the latest and greatest API, taking advantage of new features and bug fixes. That's a good thing (tm) for users.

Contrast that with Android, where developers are still targeting Gingerbread and are unable to take advantage of any of the bug fixes in ICS, JB or KK.