It isn't, if 3d6 successfully takes down the target and that's what you wanted to do. If you don't want to take down the target on the swing, it doesn't matter if it's 3d6 or 6d6. If you do, and 3d6 is enough, then it still doesn't matter. If, however, you do, and 3d6 either doesn't or can't do it, but rolling 3 more d6 changes that form "He get's hit and stays standing the swings back" to "He takes it right to the temple and topples like a house of cards.", then 6d6 was that much cooler.

A stream lined example but I don't feel like writing out a massive fight scene with 2 different endings to make the point.