> But, I think "single rod-drop" will not shut down a reactor

Why do you think that? When rods are used to control the reaction rate I thought it was by raising and lowering them small amounts usually.

Being able to have a single fully inserted rod stop things would certainly make it more likely dropping rods into the core would stop an even than if you needed (say) all 3 to be fully inserted.

3x the chance of successfully stopping a serious problem, 1/3rd the chance of a blockage prevent the emergency shutdown.