My opinion is that merit pay is a great idea in theory but may be a lot harder to do in practice.
There is no reason that teachers' pay should be based on credits and seniority. Ideally, we should pay the teachers who do a good job more than those who do a mediocore job. That's how most jobs work, so why not teaching?
The problem, though, is that it's awfully hard to figure out an equitable way of identifying the best teachers and the worst teachers. You wouldn't want to base it all on principals' evaluations or on student test scores. So do you go with some of each? Do you try to come up with some new and better way to do evaluations? These questions need to be answered, but that will take time and meanwhile there's this push going on...
Very complicated issue, in my opinion.
I also agree that this is a very complicated issue. If I am the teacher who is really good at managing EBD students in a classroom of 32, and my reward is to have more of them in my class, does that give me merit points? Because I'm the skateboard club adviser, those students want to be in my classroom and almost every single one is ADHD. Does that give me bonus points on the merit system? If I am the teacher who has the learning disability students in an assisted class, does that give me additional points for merit pay? What if my test scores aren't as high as they could have been if I didn't have so many of the students who need extra help? What if I have 19 students with an IEP and the other English teacher has only one?
Do you see why the idea of an equitable scale can be so difficult to figure out? English teachers spend many outside hours grading essays while some teachers take home nothing. Does that figure in? I believe that with the push to make education a science when I believe that it is more like an art/science mixture, many questions remain to be answered before merit pay will be in any way fair to teachers.