WTF??? Are some people for real? The amount of crashes i have seen whether it has been road, mtb, trials, vmx etc and the moment when a stem turns from the impact. Now imagine that crash scenario but they didn't have a top cap? (Possibility the stem could of slides off the steerer) The situation would be a lot worse. The top cap keeps the stem compressed on the headset and bearing so that there is no play.
(Awaits for trials-forums next topic title: Do you actually need cranks bolts?)