When my mom's MPS elementary school was deemed as 'failing' back in the 90s, she began a journey into each and every magic fix in education, which would last the final decade of her employment as a teacher and generally leave the school in as bad or worse of a state as it had begun. Voucher schools are the next step in a system of trial and error that has been created by those who hope against reality, but it's worth a shot like any other potential fix.
First, the reality. Most fixes are temporary or work on the kids who would have performed better in the traditional setting, anyhow, like those with parents who bother to sign their kids up for the better school. This should be obvious, but those in favor of the next big thing tend to use the stats, anyhow. On top of that, improvements with most of the fixes, including school choice, seem to be immediate but not lasting. From the studies I've seen, scores go up for a few years, gaps are never closed, and scores fall back eventually. However, the truth is that voucher schools have not been fully implemented in order to fix the 'problems' in education, and since everything else has been tried, we might as well try voucher schools.
Teachers who argue against the ultimate trial and error (which will result in the demise of public schools) really should have thought about that before they bought into every new program, before they accepted the 'fact' that students today are dumber than students in the past, and before they accepted their own union acceptance of teacher accountability. It's too late to go back, and we're close enough to give it a shot, so it kind of makes sense to go ahead and see what happens when all parents get vouchers and get to choose where that money goes.
While voucher schools will not be as effective as the eventual (and only legitimate) solution to failing inner city schools -- boarding schools -- a voucher system is relatively cheap as an experiment. And who knows, maybe the psychological benefit of feeling like a real choice was made will be enough to overcome ignorance and poverty. I had a planning professor in college who said people treat their properties much better when they own rather than rent, so if you own your kids' education, maybe there's college-like effect to the whole situation.