The initial post was framed as a question about the use of fixed gear bikes by messengers, and I responded in that context.
Considering the riding conditions I see bike couriers contending with around here -- hellish weather, awful agressive car drivers, major erratic traffic patterns on busy downtown streets, major time pressure to deliver, pond-sized pot holes, being stoned most of the time, etc. -- I'll stick to the opinion that this is insanity born of careless youth.
I've ridden fixed gear on a velodrome track many times and so can concur that it is a grand experience, provided one does not have to deal with the aforementioned mortal hazards.
Emanuel Lowi
Montreal, Quebec