Right, but they're not skidding or otherwise applying big impulse forces to the cog, especially against the threading.

I don't know that I buy Shants' critique of the 6 bolt disc brake system versus the level splines. I'm not exactly a pro downhill MTB racer (unless hill bombing for beers counts), but as far as I know shearing bolts off is pretty rare on those systems, and locking up a disc brake puts larger forces on the rotor than anything buffered by muscle and knee cartilage ever will. Also, shearing the bolts is probably a good thing, as the next most likely failure mode would be catastrophic hub asplosion. Also, with the level system the hub splines taking the shearing force are Al while the cog is steel, which is not exactly teh recipe for durability, and again, has a pricey failure mode.

edit: it should also be noted that the first bike disc brake designs used a reverse thread lock ring like a track hub. Not surprisingly, these stripped out a lot, and the 6 bolt rotor system was developed to replace it.