i personally just don't think the customizer is user friendly enough to start putting multiple layered menus in. Having to click, then slide then click then slide Etc etc is really annoying. It's easier to just have one screen that shows everything.

I don't mind having things like Jesse said, background image, etc, but one you start adding extra windows and extra clicks to do something simple, it starts to become too cumbersome.

If it was a full front end solution then awesome, but it's still back end with a big preview window and a small pane with everything squeezed in. Moral: just keep it simple, not everything has to be in the Customizer