It's worth pointing out that Microsoft primarily invented this for HTML5 Windows 8 apps (along with some interesting, if incredibly underbaked, JavaScript view libraries to go along with it). That is why the APIs here might seem like way-too-high-level abstractions - they were trying to encapsulate the UX features Windows 8 had, like scroll snapping within item grids.

It was nice, and necessary, to use there because it made it relatively easy to make HTML5 apps that looked and felt like a native counterpart on the same platform. I don't know if they would have fit in on the general web, but it was cool to see them take a crack at nicer touch scrolling in CSS.