As a teacher I have to say I think that it is an incredibly common problem for every school to have someone that isn't being as effective of a teacher as they could be (read in some cases awful and idiotic). In my experience it tends to be the teachers that have been in the system for a long time, most likely tenured, who are protected just by the length of time they teach. Many times these 'teachers' spend their time as place holders, completely ineffective as teachers, and often times as people that pull down the morale of the whole teaching staff. Knowing that someone is a terrible teacher, but gets to continue to teach year after year, doesn't exactly inspire confidence in the schools to many people.
That being said, in my experience it's ONE teacher in the whole school (at most two in a large school). Not a problem at a level that should cause people to lose hope in the system. They're there, and we're unfortunately generally stuck with them unless they really screw up. Which happens sometimes. But, for every one case like this we get so much bad press that it causes people to make their snide comments about the rest of the school's employees. Myself, and most of the teachers I know, all give more time to these students than anyone ever acknowledges. There's no right answer right now, but the evaluation systems in education are definitely a spot that needs re-examined in many places.