In reply to Nick Halsey.

> When used properly..

And exactly here is the problem. In real life people like a secretary etc. want/have to edit their CMS in a productive way, they just want to add, edit or remove content.

They do not want to scroll through screens-high tiny sidebars with barely understandable shortened button texts, no additional information and a useless preview which takes most of the screen.

Just try my random header images example. Or my widget/sidebars example. The customizer is completely useless for these setups.