Is Gov. Andrew Cuomo right to demand changes in teacher evaluations, or is the union right in fighting him? Or both?

Cuomo pushed through a state budget deal last week that made it harder for teachers to automatically receive the highest possible rating under a new evaluation system, and charged state education officials with working out the details. Whatever you think of standardized tests, something is amiss with the current evaluation system. Only a tiny percentage of teachers are fired each year for incompetence. As many of our schools continue to struggle, 96% of teachers were rated "effective" or "highly effective" last year.

Unions exist to protect their members. It's only when that mandate collides with the public interest that battle lines are drawn.

"The teacher evaluation system imposed by Governor Cuomo is a sham," chided the NY State United Teachers. But Cuomo believes it's the current system that's "baloney."

Do teachers, police and other public unions sometimes shield incompetent rank-and-file members? Absolutely. But students and civilians shouldn't suffer because unions protect the unfit. This has led to a growing outcry against unions -- now seen by many as another special interest group.

But while we hold unions accountable, we should not forget their value. Many people are unaware, as they take sick and vacation days, that unions were instrumental in getting those benefits. Eliminating sweat shops, establishing safety standards and a living wage? Ditto.

The rise in union membership after World War II was a huge factor in the growth of the middle class. It's also no accident that its decline (from 20.1% of American workers in 1983 to 11.3% in 2013) has coincided with a surge in income disparity.

Bullying unions has become cheap sport for some politicians, perhaps reaching its lowest point when Wisconsin Gov. Scott Walker compared his fight to take away teachers unions' power to bargain with how strong he'd battle Islamic terrorists if elected president.

Do the goals of unions sometimes clash with the best interests of the public? No doubt, and it's up to elected officials to look out for the greater good. There's also no doubt that weakening the labor movement weakens us all.

Playwright Mike Vogel blogs at newyorkgritty.net.