Depending on whom you ask, tenure is either saving the teaching field or killing it. A California court has just ruled that the California tenure law is unconstitutional because it deprives children of good teachers. Personally I think tenure has lived a good long life and now it is time for it to slip peacefully into its grave.
First, some history. Tenure came at a time when teachers were hired and fired based on lots of things, many of them having nothing to do with the ability to help children learn. A teacher could be hired because the right political party was in local power and a contribution had been made to the winning candidate. A teacher could be hired because she (and almost always a woman) belonged to the right church or had "good" family background. Likewise, teachers could be fired for being in the "family way" or even just getting married. If the parties switched control teachers would be fired. If a teacher taught a lesson that was thought to be against the mores of that community, out she went. So teachers needed the protection of tenure to make sure their jobs were not sustained by the whims of those currently in power.
Fast forward to today. Between tenure and the teachers' unions it is almost impossible to terminate a teacher. The procedures are long and arduous and if even one step is skipped, administrators must go back to step one. Add to that the fact that a teacher can request a transfer to another principal who might not be as methodical in following all the steps. I could tell you some stories of tenured teachers that would make your taxpayer dollars curl up into a crumbled ball, yet on they stay, getting raises year after year for just living and breathing. Teacher quality has no current place in teacher salary. Tenure protects the good teachers and the bad teachers. But let's be honest, there is a shortage of good teachers and there are very few great teachers- these people hardly need protection.
There is no longer any issue of firing someone because of marriage, religion, pregnancy or even in most places sexual orientation. We don't need to worry about teaching blasphemous curriculum or protecting academic freedom because curriculum is so controlled in public schools teachers complain about how few choices they do have. Did anyone say Common Core.
So what is the remaining purpose of tenure today? Simply to protect the weakest links from being removed from the teaching force. Somewhere along the line we have totally lost sight of the purpose of schools and teaching. It is for children to learn. Clearly children learn better with good teachers. It is unconscionable that we are using tenure to protect the weakest teachers while sacrificing children's learning.
I am sure the teacher unions have other reasons for tenure, I just can't think of them