Originally Posted by SyntaxPC
I don't think anyone has mentioned this yet, and it might be out of the scope of your question, but one can ride brakeless on a fixed gear but one cannot (as safely) on a SS (although I know some crazies that do). Intuitively, you're going to burn a lot more calories and exercise a greater variety of muscles by letting your legs do most of the stopping, as opposed to letting friction do the stopping.
I was going to bring this up. I have a front brake on my fixed gear, at first I was using it a fair amount, especially if I was already tired from riding. Now that my muscles have built up, I use the brake a lot less, and I don't feel like it's that hard to resist/skid/skip, but it's definitely more work than hitting a brake lever.
But I think the "better workout" argument is lame anyway. I've had people comment that my bike is so light, but then say they don't mind that their bike is heavy because they could use the workout. Of course, then we ride and I have to go real slow for them. Are heavy bikes better work outs? Not if you're only going 10mph on them. Push yourself if you want a better workout, a bike is only a tool.