But when are you really going to overboost the engine? You are using autothrottle and the engine will be commanded to stay within limits. We had a situation a while back with a failed SAARU using the MEL. After start, as expected, both EEC's were inop. Full thrust takeoffs for both flights at light weights but no other real worries.
In this situation on the 74, when you disconnect autothrottle for a normal landing, you are very vulnerable if you do a go-around due to manually advancing thrust. But on the 777, the autothrottle is left on until the landing. So if you go around, TOGA is pressed and the throttles move forward automatically and respect the limits(except if the go-around is after touchdown or is it below 2 feet RA).
Also in alternate mode, you now have more power available to get out of a windshear or GPWS scenario if really desperate and you want to push the throttles to a higher than maximum rated power setting. I suppose a stall/low speed scenario could also be a factor.