For one, Gardner seems to be saying at the end of his op-ed that the students left behind by vouchers are actually worse off with vouchers than without, writing,
Without anyone in their corner, children who remain in regular high schools have a graduation rate well below that in voucher high schools
...
voucher supporters believe these children constitute the unavoidable price that must be paid in the service of the principle of choiceHe cites their poor graduation rates, but this evidence doesn't prove that point, since these students would probably do poorly with or without vouchers. I would imagine that these students would be better off with vouchers, since they would be attending smaller public schools, which could thereby serve their smaller student pool better. Also, since vouchers are considerably cheaper (saving 10-15 thousand, and even higher in DCs school district, PER student) then that frees up more money for these students, though, of course, more money per se, is not the route to better education ( though it can potentially open up more possibility for better education).
Gardner is also implicitly arguing that the kids who take advantage of the vouchers are not better off, evidenced by lack of improvement in test scores. This is also a weak argument, since test scores are a weak indicator of good education. Long term success is a better indicator. This is a doggedly difficult thing to measure since its ultimately impossible to tell how successful they would have been without the vouchers. Thus, that parents of voucher kids are happier with the program is probably the best indicator available, and it shows that the vouchers are good.
Thus, if Gardner is making an argument that doesn't fall into the Nirvana fallacy, it seems to be that vouchers don't help those who do take advantage of them and do hurt those who don't take advantage of them. I think both of these arguments are weak and are poorly supported.
Of course, Gardners primary argument is that people are wrong who claim politicians are being hypocritical by sending their kids to good private schools, and then trying to deny the vouchers that are the means of the less privileged going to good private schools. But this argument depends on showing that vouchers actually hurt the poor, which he fails to do.
In the end, I think vouchers are a good idea, mostly just on the grounds that they save oodles of money. I think an even better solution would be to eliminate public schools and vouchers. Behind vouchers are the threat of creating arbitrary and ineffective standards for schools to be eligible to receive vouchers. Without vouchers and public schools, education could serve students instead of governments. Poor youngsters would be better served since most likely this situation would create the existence of free charitable schools for the poor, which could serve those who couldn't afford even the most inexpensive private schools. And I think most likely such schools would be guided by a mission to actually recruit underprivileged and neglected children and try to bring them to their school. The problem is that when the government swoops in and takes responsibility for something, like educating the young, it lifts the burden of people to take personal responsibility for themselves and their community. You remove government and people will be trying to assure that the kids in their community are educated.
Update: Mike Smith, senior advisor to the Secretary of Education made a statement about whenever a decision is made about education curricula or standards, at the federal level, it will be inevitably guided by politics. The Cato Institute responds that whenever a decision is made at any level of government (federal, state, local) on education standards and curricula, it will inevitably be guided by politics. And they link to a paper Neal McCluskey published for Cato two years ago called Why We Fight: How Public Schools Cause Social Conflict, that argues the only way to avoid conflict about curricula and standards is to let parents choose their schools.