I said " it wont do it automatically when it drops below the set point". I didnt say "it wont do it automatically".
What I mean is, if you have the car set to 70%, it will not automatically start charging when it drops to 69%. If the car was parked for several days, that might cause it to charge a lot more frequently (69%, 70%, 69%, 70% etc). It doesnt sleep when it charges so that would keep the car awake more, unecessarily.
As I mentioned, I believe it starts charging after it gets about 3% below the setpoint. As for the "why keep it plugged in?" Other than to say "Tesla recommends this, but its up to you" I am not addressing that. There is tons (and tons and tons) of discussion here on that discussion point though, so you should find plenty of discussion (but nothing other than discussion) if you search for it.
No, there is no way to keep the car plugged in at a set point below the bottom, if you want to store it like that for some reason, you will need to leave it not plugged in.