Merit pay for teachers improve our education system in many ways. The give the teachers who do a good job to do an even better job. Students then benefit from those teachers who put in extra effort to provide a good education. Even more, teachers who slack and don't care about their students or their job don't get any incentive.
Merit pay would drastically improve the current education system. Right now, teachers are often in unions that fail to hold them accountable for their results. Merit pay would not break the unions that most teachers favor, but would allow good teachers to excel while poor teachers would seek more lucrative careers.
It is sometimes not the teachers fault that the student got a bad grade on a test. There could be something at home going on, or the student could have some kind of disability. What if there is a teacher that likes to get in-depth in what there are teaching? And tests are not always correct. There are many reasons why there should be no merit pay.