What I don't get is why for some reason everything that includes a "Live" preview should be tucked into the customiser. I can absolutely see the benefit of having a live preview of your new menu before you put it live. So what about just doing what WP has been doing for years with Post previews? Why don't they add a big nice, shiny "Preview Menu" to the existing (incredibly well made) Menu manager that shows how your new menu will look? And what if this preview window would automatically refresh whilst you are working on your menu. You could dedicate a full browser tab/window to having constant live previews without having to cram all of this advanced functionality into one screen.

And if that previewer works you can look at using some of the awesome JS stuff in Core to load this preview through a fancy animated JS/AJAX preview. This way you could move towards a preview that does not require a second browser tab/window to be open. Like this: http://codyhouse.co/demo/animated-page-transition/about.html

What I'm trying to say here is.. It feels and seems like the Core team has decided in order to move WordPress forward it needs to make things more clear and easy to the user through showing changes as they happen. I think this is a great idea but hinging all of this on the Customiser simply because it's the only part of WP that currently offers this type of live preview functionality is troublesome.

Please reconsider this way of moving forward Core devs. I think it's a mistake from especially an user experience point of view.