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Yeah I'm not sure about road riding - only have a front on my road fixed - but I have done FGCX and been happy many times to have had a rear as well. It was helpful with balance and cadence especially at slower speeds and down slippery hills, so I'm not sure I'd say having a rear brake on fixed is counter-intuitive. Maybe just a bit redundant, but everyone descends their own way (especially fixed).
Don't mean to come across as a dick but if you want a rear brake to control your cadence and you can't manage the speed of your bike with your legs (and if you use your front brakd often), I don't see a point in riding fixed. It's counter-intuitive. A single speed cog would be ideal for that kind of use.