I tend to think of this as being more like network flow control. If the server is overwhelmed with requests it can tell the client to back-off for a bit. Sure the client can keep on hammering the server, but then everyone loses. If the power grid goes down most lose more than a few degrees of comfort.
It also seems silly to create excess capacity for a few peaks during the year. Perhaps they can get away with batteries or other energy stores instead of power plants. However, if the extra capacity/storage is only needed for a few days of the year and the expense can be avoided using a voluntary mechanism, why not implement it?
Additionally, think of this as using individual homes as storage banks. Perhaps the power company can anticipate demand and overheat or overcool your home allowing for more room to cut power later on.
I think the voluntary nature of this arrangement needs to be maintained. What if you decide to start up your home Beowolf cluster on the hottest day of the year.