There are strong teachers and there are weak teachers in education - no argument there. That might be the problem but the solution is not to identify those weak teachers and then pay them a substandard wage to drum them out of the profession. The problem is systemic and has been a part of education for... ever. After graduation, there is no system for lifelong learning.
The problem starts in the beginning of a teacher's career. The problem is that after teachers graduate from college, their learning stops! There is no system in place to make sure that a beginning teacher has the support that she needs in those first years to figure out how to put that book knowledge that she has gained into practice. Yeah, we give beginning teachers a "mentor" but in most cases that comes with no release time and really just means it might, or might not, be someone you can ask a few questions and who might check on you every now and then. Good teachers search out a real "mentor", someone that they can align themselves with. They watch her every move, get into her classroom as often as possible and ask a million questions. But that's not a system, that's an individual teacher figuring it out on her own.
Not only that, there's not an improvement model for teachers in the midst of their career, when they have the basics under their belt, to grow and learn, so they just continue to do what they have always done - good or bad. They might get a new little nugget here and there and if they have the money and time, they might attend conferences and really seek out educational opportunities. On-line opportunities abound for the eager learner, but it's not easy. There is no system to help you navigate the opportunties or encourage you. You often pay your own money and spend your own time for benefits that are self-motivating and self-gratifying, but not necessarily rewarded monetarily.
As you move into the sunset of your career, I guess everyone just assumes you already know everything. You've had years of experience, but if you've simply been repeating the same things year after year, without growing, are you really any better? There is always so much more to learn.
An evaluation system might hunt out the weaker links in our schools, but a better way might be to put the time and money into quality professional development offered in an array of opportunities that could be self-directed or even self-designed. If the money and time being put toward designing evaluation systems could be put instead toward providing quality, empowering professional development, then the changes would be tenfold.