I've never liked a rear brake on a fixie (and I have tried) because it's extra weight, arguably a redundant component, and is a real pain if you have track ends, because the rear brake pads need adjusting every time you retension the chain or change sprocket size (it's not always necessary with "horizontal" road ends, which aren't actually horizontal for this very reason). Also, and this is a personal thing, I always forget to let go of the rear brake lever if I stop with the pedals in the wrong starting position and need to lift the rear wheels and turn the cranks, which is just embarrassing.
Having said that, I'd have a rear brake if I rode mainly in hilly country, because of the brake heating problem.