I don't think you understand how the snap feature works. When a Metro app is snapped, an event is fired informing the app of which location its in. This is a more precise configuration than a simple resized event, and it allows the app to shift its view to be more presentable in that format. This allows developers to create distinct views for snapped vs full screen, which, if done properly, will both look better & require less dev effort than parsing through resize events.
It's still less flexible a system than traditional windows. I'm hardly going to tell you overlapping windows is a perfect solution, but it's been working just fine for 20 years. I can envision many things they could do to improve it. Scrapping it entirely for a oversimplified mode is going too far.