I don't much to add to this argument except that it does seem silly for middle class folks to be arguing against vouchers that give poor urban students the choice that middle class folks have made by living in a more affluent neighborhood.
The issues are not about the actual schools or the kids or even the parents. What vouchers do is give people a choice. And Choice is what America is fundamentally all about.
There is no law that says there has to be a public school system, all the state constitutions say that the state shall provide for (as in fund) a system of public education, it does not say that the state shall operate a system of public education.
"Public schools" are a consequence of legislatures wanting a little more control over how education dollars are spent, but there is nothing magical or required in a "public school." Indeed, arguable private schools generally do a much better job because they are far closer to the cusumers of educaiton (namely the children and parents).
Oh, wait, that is too "market place" for liberals. My bad.