I have a formula fixed/fixed hub, with a 48x16 gear, and for some reason my lock ring likes to come off, taking the threads on the hub with it. I've been told by my LBS that its because my lock ring is steel and the hub is aluminum, and because steel is harder than aluminum, the threads on the hub tear out.
They way the hub is designed with opposing thread patterns, if (big if) the cog were to slip back for some reason, there's no way for it to undo the lock ring. I find it hard to believe that my legs are strong enough to force the cog to unscrew, and take the lock ring (with its opposing threads) with it.
I'm just having one of those days but really, the threads for the cog are made of the same aluminum as the threads for the lock ring.