"One-and-done" may make sense for States and Regionals, but it makes absolutely no sense for Cities. Cities are about as informal a setting as one can get in a Premier tournament, since there are so many of them. (The only thing that could be more informal would be Prereleases.)
Speaking from personal experience, Cities are a time of experimentation, a time to see what works and what doesn't in the current archetype, a time to hone one's skills, and a time to pick up some major bragging rights. If Cities became "one-and-done," then most of this would be lost. (However, if I ever won a City Championship, I would take it easy for the remaining Cities in my area, just to let others get a shot. I'm simply that nice.)
- Croatian_Nidoking