As has been mentioned, the differences are not great. However, Fixed-gears require less upkeep in terms of drivetrain adjustment, and if you ride in dirty conditions the chain stays cleaner because it doesn't hang down so low since there's no derailer.
And carrying a 15mm wrench to remove track nuts in the event of a flat tire is a pain. I do it with my fixie, but not with my commuting bike (geared but with track nuts as theft deterrent) because I'm rarely very far from home on that thing and don't carry tools anyway.

As to why messengers use fixed-gear bikes - this may shed more light on your question.
- They're cool and hip and show that the rider is "serious" in a sort of hipster way
- they can be tossed around more, fall on their side without worrying about banging up the derailer, and they won't drop a chain.