It takes money away from public schools and diverts it to private ones. Private schools can pick and choose as to whom they want and don't want. The public schools will be left with the students who require more because of special needs and will have less money to meet those needs. There are also some whose parents were not able to endure the expense of private schools. I could see the difference in their attitudes about school, and also the things they were learning and at what rate they were able to learn things.
Private schools know better how to spend their money.
Public schools have too many politics involved. The private schools that I know about are already maxed out. Proponents refuting that public schools would not become a dumping ground for underperforming children and that it is not in their best interest to discriminate on the basis of race, ability and gender fall on opponent's deaf ears. A strident, loud and persistent harangue from both sides doesn't mean either side is right. Sadly, few are asking the student's opinion.