This option appears to be available for controller users, but not for mouse users.
I think this was a great idea, and would really like to use it on my PC/mouse.
An even better mode would be UI when weapon is unsheathed, and a UI setup when weapon is sheathed, that way I can enjoy the beauty of the FFXIV world when I'm just walking around, and have the UI I need when I get into a fight!