But ok, you go right on ahead believing this is all about unions protecting incompetent teachers. (Yes, incompetent teachers exist, so spare me your anecdotes. Incompetent police officers, doctors, firefighters, pilots, accounts, lawyers, etc. also exist. It's called life.)
what not one of the union demonizing, canard quoting people can tell me is why would a union want to protect the poor performers?
Do you know who runs the local union? The folks from the same employer elected by local union members - not some nebulous union-boss bogey men in fedoras. Why would the folks that do their job and do it well want to protect people that aren't? Poor performers make more work for the people that actually do their jobs or make the whole group look bad - They'll resent the poor performers, not want them protected.
What a union will VIGOROUSLY do is protect the contract by making sure that the contract is followed - particularly language in the contract regarding discipline and dismissal. I guess to those with an anti-worker agenda this could look like protecting a bad worker, but it is making sure the rules are followed regardless of who is in the hot seat.
When I was chief steward there was an awful worker we all would rather have seen gone, they were a constant source of stress for everyone else with the poor attitude and performance that poisoned what ever office s/he was placed in. Why did s/he last for years? Because of union protection? No, it was because the so-called supervisors didn't want to follow the steps outlined to get ride of a poor worker - it was easier to transfer that loser around to different departments. S/he stayed not because of union protection, but because management refused to exercise the "management rights" they had enshrined in the contract.