I wouldn't want to disable the menubar. I want it hidden most of the time, but I want it easy to unhide. The current implementation works very well for that.
Same thing with some other UI elements. In a large game I hide the playerlist and unhide it when I want to check the scores.
In fact, while I'm spectating, I usually want to hide the command panel and selection panel. There's no easy way to do this, so I bound a hotkey to the 'luaui togglewidget Chili Integral Menu' command to enable and disable the widget. It's kinda clunky but it works.