I'm a developer and I do use the Customizer when setting sites up for clients--but mainly for things that have been moved there, like site icon, colors, and header images.

I don't do either widget or menu management with the Customizer because it's too clunky. (Mind you, widget management is clunky regardless and menu management is clunky if you have more than a few menu items--can we PLEASE have a way to collapse submenus?)

I do use the new Additional CSS to test changes to the CSS, which I move into a proper stylesheet when I'm happy with them.

I'm not sure most of my clients know the Customizer exists. They've generally hired me to do the customizing and most of the questions I get from them are about editing posts, pages, and CPTs.

Because of the non-representative sample size and the way the survey was written, I'm not sure we can really conclude ANYTHING from the results...except that we need better methods of collecting user data.