They still work as libraries. 'Pure arduino' with it's setup(), loop(), pinMode(), etc is hiding a lot of firmware from you. If your device has wifi, it becomes more difficult to use it without an OS since you must manage the wifi yourself inside your loop(). Failure to respond to radio events within certain time frames is an error, so you would have to be careful to not spend too much time in your loop before dealing with the radio. With an OS, the radio has it's own thread and you don't have to deal with it. To you it works like a library.
