Coming at this from the higher ed side...Yea-let the parents and administration micro manage teachers more than they try to do already with no union to back you up. Teaching is a whole different ballgame that you can't possibly understand unless you have been in education.
I was never more happy for a union when one particular administrator turned against me and made my life miserable including incessant micro-managing and holding tenure over my head.
What the union did was step in and put pressure on this particular administrator to follow the rules and written procedures...nothing more and nothing less. Yes, the union was ready to fight, but really all it took was someone from the state-level union meeting with the college president and outlining what had been going on. After that, I started getting objectively evaluated, was held to the same standards as everyone else(not make up rules as you go) and actually ended up getting a promotion out of the whole deal. Aloe bng the way, the union connected me with resources to help me be a better educator.
So, that's what an education union did for me...