Nobody might care about the 3D cube but stuff like shadows under windows have a clear usability benefit. aka: knowing which window is on to of each other. Of course in a tiling WM you don’t care about that but vast majority of people don’t want to use tiling WMs.
Why would I need to know the Z-order of overlapping windows? None of the windows I'm actively working with overlap (it would be too annoying to have them constantly blocking each other), and all the rest just have "background" status. I don't care how far they are into the background, so drop shadow is useless visual clutter.
You are you. Other people work differently. If you want to hear how a power user uses overlapping windows I’d recommend listening to this ATP episode: https://atp.fm/96
Shadows don't need a compositor though, do they? At the least, affordances don't need a compositor. So, make the "active" border less soft and this is easily doable. (I feel like this is how things used to be?)
