Agreed....mostly. While teachers can improve, either through observing other teachers, guidance, etc, the issue then becomes how many student's years of learning should be ruined for it?
If your child was in the class of a horrible teacher, would you keep them in it so the teacher could practice teaching with your child? Let them get through their learning process and hope that if/when your younger children have the same teacher that they have improved? Or would you take them out and put them in a class with a better teacher?
It isn't as simple as waiting for the system to vet out poor teachers, there are individuals that suffer from poor teaching. I wouldn't want my children set back a year while waiting for a teacher to figure out what they are doing.