I recently had to ride without my rear derailleur for one morning commute (broken cable) and I absolutely hated it. Even though Shanghai is flat as a pancake, there are a couple of bridges that have to be climbed on my route, plus the wind seems to be quite variable, and so I change gears relatively frequently. I just couldn't get by on a single speed. Mechanically simple, maybe, but not being able to keep an efficient cadence in any conditions pretty much ruins the idea for me.