This argument is obviously true. But, good luck getting anyone who is already brainwashed to agree. I recall a lunch discussion with an alleged MENSA member and another individual who I consider to be one of the most intelligent persons that I know. At the end of the discussion, the only argument that they were left with, was that the government can not be trusted to adjust its fixed expenses. AAArrgh.
The chief opponents to vouchers are the teacher's unions and politicians beholden to their lobbyist. There is obviously a "conflict of interest" with respect to those who are chiefly opposed to them, plus those who have been brainwashed, mostly by the public schools themselves. One of the beauties of vouchers are they break this monopoly as well as what I would call the "secular religion" which permeates the public schools. There'll be many who'll argue that there is no such "religion", but if so, why does it have many of the same characteristics which define a religion (e.g. an ideology, a set of values which aren't embraced universally, etc.)
I think the next step should be allowing public funding to private schools including parochial. As long as such schools can meet certain accreditation requirements (e.g. I'm obviously not speaking about about schools preaching death to the infidels, or black's are racially inferior or superior etc.), why should the "separation of church and state" arguments be a continued impediment? The issue of getting prayer back into public schools is never going to happen, nor do I think it should. The beauty of voucher's, and one that public funding of private schools have in common with it, is that it makes an end-run around the "separation of church and state" argument; which cannot be said for the getting prayer back into public schools argument. An additional beauty of having a similar commonality is that vouchers have passed muster before the "Supreme Court."