I think it would be nice to have as an option, especially when travelling but I also do think that most people don't really want that and prefer the separate device paradigm.

I don't want to toggle options and stuff when I don't want to be distracted or interrupted with calls or messages, I just leave my smartphone in another room. Sure you could attain the same with some keyboard shortcuts but you are pretty sure to sometimes forget to do it while not bringing your phone with you is inconscious and always work.