For singlespeed bikes, what are the practical pros and cons of a fixed gear vs a freewheel for transportation?

When I say practical I mean practical. I don't want to hear any bull**** about it being more fun and I don't want to hear anything about how fixed gears improve strength/cadence/etc. The reason it improves your strength/cadence is because it makes you work harder to achieve the same result. That is a practical disadvantage, not an advantage. An indoor cycle is good for training too, but it couldn't be worse for transportation purposes.

The reason I'm asking is because it seems to me that (when all things are considered) fixed gears really aren't better than freewheels. However, this doesn't make sense in light of the fact that fixed gears are popular among bike messengers. Why would someone who spends all day cycling and whose livelihood depends on their ability to efficiently cycle pick the worse alternative, especially when switching to the other alternative is as simple using a freewheel instead of a lockring?