A few years ago, Jerry Hausman showed that Wal-Mart does a lot to benefit even consumers who don't shop there. When a Wal-Mart opens, competitor local supermarkets cut their prices to keep customers. And poor customers reap most of the benefits.
Figlio and Hart, in the latest AEJ: Applied Economics, show a similar effect with school vouchers. An ungated version is here.
Suppose your worry about school vouchers is that low social capital parents' stick with a local underperforming school while kids whose parents have better social capital all flee with their vouchers to the better private schools. And suppose further that you care way more about the potential losses to the former than about the gains for the latter. You might then oppose voucher systems.
Figlio and Hart show that public schools facing competitive pressure from private schools under a new voucher system provided stronger student score improvements. All that concern about kids left behind as the private schools cream off the best voucher kids? Not much of an issue if the public schools facing the competitive pressures perform better as consequence. They find the biggest positive effects in public schools facing strong financial incentives to retain low-income students.
Their identification strategy's pretty decent, exploiting the timing of the voucher roll-outs across the state. But do go have a look to see if they've accounted for your particular objections. And then update your priors.