True, but one person can use account on two devices also.
There are differences b/w kite, Pi and Nest. What if I want to chart on Pi and place through NEST etc
Also, single login doesn't guarantee that account is logged in only by account holder.
For this, they have two step auth in place already. They can strengthen it by using something like Google Authenticator etc which many have started implementing as well.
Every 30 secs, it refreshes.
I was just going along the lines of gmail, skype, fb etc and they all have a unified system. Infact, based on other type of activity detection systems, they occasionally send notifications of suspected unauthorized access.