It's consistent w/the logic of choice. So it does have a rationale in policy.
Politically, it's a way to provide a limited number of tickets to suburban schools for low-income, minority urban families. It's a great way to embarrass urban school systems and discombobulate Democrats.
If we could read minds, we would know precisely how much each motive governs the policy.
I personally doubt conservatives are serious about vouchers, beyond its considerable rhetorical and political effect, since the implication is to breach the boundaries of local government. An exception would be some religious conservatives, for whom vouchers would be bonanza since it would defray the cost of education for parents who use parochial, other religious schools, and perhaps even home schooling.
mbs