Teacher Residency Requirements

Apart from being marginally good local politics to require city employees (including teachers) to live within city boundaries, why would an urban district create barriers that make it more difficult to attract the highly effective teachers that it needs?

Ask Chicago and Milwaukee. (Boston, too, has a residency requirement for city employees, but it excludes teachers.) Any others out there we should be aware of?

From the Chicago Tribune (9/11/2009):

The city, for its part, maintains that teachers should be contributing to the tax base that funds their schools, among other reasons.

From the Milwaukee Journal-Sentinel (1/24/2008):

The residency rule has been controversial for years. Some say it is unfair and MPS needs good teachers too much to restrict the pool of possible teachers. Others say it doesn't actually have much effect on who teaches overall and it's good for the city to have employees live within the city line. Efforts in the state Legislature to repeal the residency rule recently have not succeeded.