Today per account you can only have one device playing at a time. So if you have your account playing on your home speakers you can't also have it playing in your car. or if you have it hooked up to a bluetooth device upstairs you can't also play on a different bluetooth device downstairs.
Allow multiple symultainious logins with the same account. I would pay extra, I just don't want to have my music turned off because someone else turns on another device somewhere else in the house.