Before you get worked up over this topic, realize that there are some teachers who are exceptional and others that are quite bad.
The main difference between teaching and other fields is that once a teacher is hired, it's very unlikely he/she will ever be fired for poor performance.
As a result, many schools contain both a few exceptional teachers and a few real duds. Sadly, the exceptional teachers are paid based on seniority just like the duds are, and some duds have been there a long time and are earning decent salaries and very generous pensions.
I don't think anyone would object to paying exceptional teachers more... the problem is that we all know of some really horrible, lazy teachers whom it would be a travesty to pay more.
Sure, if someone waved a wand and doubled teacher salaries, the quality of new teachers entering the profession would increase, but this would not address the vast majority of existing teachers.
It's not complicated... the only reason any firm is able to hire and retain skilled employees is because skill is rewarded compared to mediocrity. This is not the case for teachers, except in the form of non-monetary rewards (often exceptional teachers are greatly respected, but this doesn't pay the bills).
As long as we pay teachers based on seniority and other backward measures, we'll have people who are just not cut out for teaching responsible for educating our children.
I'm not advocating any sort of across the board measurement of what constitues success. This should vary by district. The important thing is that just like any other profession some people should fail to make the cut and should find work they are better suited for. As it stands, we give these people an incentive to continue teaching our children. It's no wonder they resort to trying to strengthen their union... they certainly aren't getting any rewards from being bad at what they do.
Note, I'm talking about the 20% of teachers who could be considered poor. I'm not all that concerned about the ones who are average or above average, since they are not the problem.
The issue of pay is a red herring, btw.