I am currently making a quest that has the subscreen disabled completely. It is a little annoying going through every screen and setting the No Subscreen flag. It would be really nice to be able to set this for every screen on a given map. I am sure it would be useful to people who use that option for cutscenes. They could easily dedicate a map to cutscenes and not worry about setting the flag.
Haven't used these but the same might be beneficial for Layer N is background flags.