The rhetoric about "bad teachers" may never go away -- in part some teachers will always perform poorly, act irresponsibly, and so on (just as there are poor performers and irresponsible people in all professions and fields). That said, what bothers me most about the rhetoric is that it continually oversimplifies the problem. Too many commentators seem to assume that bad people magically pop up in schools to torture principals and belittle children. But reality is more complex.
One situation that has arisen in numerous anecdotes I've heard from teachers is that a Principal will allow selected teachers (often their friends) to behave irresponsibly or worse. Examples include showing up late, parking illegally, dressing inappropriately, eating meals with the Principal instead of teaching, leaving other people in charge of their class while they run errands, and, in at least one instance, abusing children.
Are "bad teachers" a problem in these schools? Absolutely -- and they should be dealt with -- but not following the script we normally read (teacher is bad, principal wants to fire him/her, union steps in). In these cases, the story I hear is that a "bad" principal allows a few teachers to do as they please while the rest of the teachers stew in outrage and cower in fear.
Did the Principal in these situations make these teachers bad? It's not quite that simple. But these Principals have certainly negatively impacted the performance of a few teachers while subsequently damaging the climate and performance of the school as a whole.
Situations like these are why I worry more about the extent of the damage done by irresponsible Principals than I do about the damage done by irresponsible teachers.