The only reason I could see turning a bike into a single-speed is because the person dis-trusts their self-control and wants to keep their speed down in and amongst high traffic and heavily-populated areas. In this case, I think it a stroke of brilliance and moderation.
I ride a 12-speed road bike, and have, every once in a blue traffic light, found myself going too fast to stop in the split-second moment that is sometimes required, here, in Boston and Cambridge.