back pedalling could cause the cog to unscrew under force, hence a lockring. since a bb lockring unscrews in the same direction as a cog, both could come unscrewed at the same time with enough pressure. that's why lockrings are left hand threaded: when the back pedal pressure makes the cog want to unscrew, that direction will only tighten a lockring. some people will locktite the crap out of the cog and bb lockring and use brakes to slow down more than their legs. i'm sure someone will say that setup works fine, but i think it's ghetto and dangerous. do it right the first time.