Never a teacher but I was a school board member in NY several years ago.
I don't know if it is true or not but I was told many years ago that tenure came about because agricultural communities would not keep seasoned teachers without it because of the cost of the seasoned teachers. This country was basically agricultural a while back so maybe it is true. In any case some feel test scores are the bench mark to measure teachers but I disagree. Test scores by themselves are not a fair evaluation. For example most of us know people who perform well but test poorly and those whose test great and perform poorly.
I don't have the answer as to how to measure teachers effectiveness but IMHO tenure does nothing to solve that problem.
Here is an interesting read.