You probably wouldn't see Apple using USB instead of their own 30-pin Dock Connector… unless there is a better solution. Two things: It's a single connector for power, audio, aux devices, etc. Going without it would mean more logic in peripherals, making them more expensive. Second, it's great for lock in, since every peripheral developed 'for iPod/iTouch/iPhone/iPad' is a peripheral which cannot be used by its competitors.
So, win/win (for apple). No one else is allowed to use it, and it's cheaper for the end user in the short run which makes for a good selling point.