Just out of curiosity, how often do you let someone else use your phone? Not check out your phone but actually use it to the point they'd require a guest mode? Guest mode on a phone is like flying lunches in via Concorde from Paris.
It disables the networking, leaving the rest of the phone working.
How much developer effort could possibly be saved by merely neglecting to make a UI for this that can fit on a small screen, as well as a large one? This should be a solved problem.