If you have a really sweet set of freehub road wheels that you don't want to relace to a track hub, it's the way to go. That's about the only reason though. It's a lighter weight and more elegant solution to just find an decent cheap fixed hub and get your wheel laced to it.

Or just pick up a whole new fixed wheelset for under $150 if you shop around. The fixxer costs like $80, so you make the choice.