Sleep tracking is one of those areas where the Apple Watch falls short compared to activity trackers. You can get third-party apps that will provide some functionality, but that's clearly not as slick as having built-in software or sleep-oriented hardware. If that's what Apple is focusing on, you could buy the Watch knowing that it would cover health tracking at all hours of the day -- you wouldn't need to stick to a fitness band (or consider owning two wearables) just to improve your rest. It's easy to see Beddit's technology helping with other devices, too, as you could get detailed sleep data in iOS' Health app.
Beddit-derived sleep features aren't likely to show up in iOS 11, watchOS 4 or the third-generation Watch. If they have sleep tracking at all, it'll likely be Apple's in-house technology. However, this at least reflects Apple's seriousness on the subject. There are issues to solve before the Watch is an ideal sleep device (battery life, anyone?), but you might just see a time where you have a good reason to leave it on when you're ready to rest.