The argument on performance comes from the idea that theoretically the rules are applied in order. So the browser has built in stylesheet of 8px, and after that a margin of 0 is calculated. (And after that, some more specific margin in the stylesheet is recalculated)

Basically * { margin: anything } adds one extra calculation to each element in the page.

Not sure if the performace hit is measurable tough, knowing how much optimization goes into browser engines.