I guess that it is just a safety feature, if the a/thrust is not acting as you require it too you need some way of disabling it for the remainder of the flight so you are not constantly having to push the disconnect button every time it has a wobbly. Same theory with the sidestick takeover pushbutton, if you hold that for an amount of time (30seconds I think) then the other sidestick is 'locked' out for the remainder of the flight, once again that would be a very handy feature if you develop a side stick fault.