Tau, Let me clarify what I meant by "adds to the game." It's the GM's job to make sure that aspects, compels and invokes benefit the game in one of two ways, the story or the overall fun.
Does this benefit the story? I'll give you a little on that one, it certainly adds a flair to the story, but it actually takes away from the drama/tension as I will explain in a moment, and tension is necessary for any story.
Does it add to the fun of the game? The first time, definitely. The players feel the rush of power, something interesting happens. That can be great. The twentieth time? No, by then it will be agonizing as the wizards handwave whole armies to death and every other player sits on the sidelines until they happen to be able to pull off something similar.
And now we get back to my earlier point about tension. When the players can instantly kill any enemy with a single roll there's no challenge, no tension or drama. These are the things that pull us, that make us want to move forward. Without them what is the point? Why would we continue to look under rocks if we don't care what's under them?
That is what I mean when I say that a compel like this doesn't add to the game (furthermore it detracts from it).