Meh, running up the score against crappy teams doesnt mean you play good defense. Their offense is their defense. They have such a proficient offense, that it makes it hard for other teams to keep up, causing them to force quick, bad shots.
All you have to do is watch the Lakers, and see that they don't shut anyone down. They go for turnovers on defense, and try to outscore them like crazy. And the style works for them, as opposed to teams like the Suns, or Mavs/Kings of old, because the Lakers are capable of operating in a half court set. That's what annoys me about when people say that an offensive heavy team cant win. Because they can, its just that teams that have no half-court offense, are the ones that run into trouble. The Lakers offense IMO is just as good as the Suns was a few years ago (not as flashy, but just as effective and efficient). But since they have players that can create their own shot (as opposed to the Sun when it was just Nash), they can still score when the game slows down.
The Lakers are an offensive-first team, and anyone that says otherwise are morons.