I think practicality is not very high on the list of considerations when deciding to ride a FG. There are some practical reasons for single speeds in general (lack of shifters/derailleurs that can break or be stolen, lighter weight), but from a practicality standpoint, a single speed with freewheel is preferable to a FG.

I think part of the current appeal is fashion, but it also arises out of fun. I have a fixed gear bike and I simply enjoy riding it because I feel more "connected" with the bike. Although I don't ride it nearly as much as I ride my geared bikes, I still enjoy the feeling.

Also, there's a sort of a macho subculture among FG riders, especially brakeless riders. They know that riding is busy traffic is dangerous, and that is part of the appeal.

Mostly, though, I think it's because we all want to be like Kevin Bacon