<i>fixed geared riders...even those that ride on the street have learned that by varying the spin you can have as great of an effect on decelerating, if not better, then if you had a rear brake.</i>

No matter what this statement is just absurd. Whatever you think about riding on dirt this is just bull****. With the gear ratios(around 3:1) and increased friction on the road noone is strong enough to do this. This is why skidding and skipping are so popular. For real world riders they stop you faster even though the codf is lower then the cosf. Unless you are a world class sprinter(and probably not even then) you are simply not strong enough to apply enough tourque throughout the pedal stroke to take advantage of the increased cosf. People claim they can control thier speed better but when push comes to shove they end up skidding which illustrates how they can't. Poeple make alot of absurd statements about fixies though so this is to be expected but the proof is in the pudding. Have you read that hilarious fixed gear harmonic motion essay by the way it perfectly illustrates the ridiculous claims some people are willing to make?

On dirt the situation is slightly different due to lower ratios and decreased friction but the theory is the same.

I bring up people spinning out while climbing to show that even with a <1:1 ratio most people are capable of breaking the tire free in the middle of the pedal stroke but not at the top/bottom. Even with the decreased weighting being able to provide enough tourque to brake free at the top/bottom, is unlikely with a reasonable ratio. Therefore you cannot provide the ideal amount of torque smoothly to the rear wheel.