Interesting question. Why would you want it to be plugged in all the time? One possible answer is so the coach battery could be charged. That also is the reason most do not plug it in all the time - it over charges the battery and boils off the acid.
Another benefit is that the lights can be turned on anytime without worrying about discharging the battery. We only plug ours in maybe one day before we leave on a trip. We save on the electric bill.