Tenure has become the tool to protect the unimaginative, uncreative, unambitious and in general the incompetent members of the faculty. Think of hiring an employee and only finding out after two or three years that the employee had no ambition, did as little as possible to perform his or her job and took advantage of every loophole in the system. As soon as you had learned that, you would fire that employee and move on. But with tenure IF YOU HIRE THEM, YOU HAVE GOT THEM FOR LIFE. Never be naïve enough to believe that you can pass them off on another university. It will not happen.
Remember that tenure creates a lifetime employment contract between the university and the professor which in all reality cannot be terminated by the university.