// When people have no valid response to a point in debate, they resort to diversionary personal insults. This merely shows up their incompetence. //
That's not what happened. Cameron was speaking, and Balls was muttering throughout it (he's moved on from the arm waving thing). Cameron lost his temper and snapped an unscripted insult at him.
I make it a 1 - 1 draw in the childish game, because Balls achieved his aim of winding the PM up until he snapped, but the laughter was so long, widespread and uproarious when he got called an idiot, that it evened up the score.