The important context for thinking about this is salary levels. Teachers are paid substantially less than other professionals with similar levels of education. They also tend to work high levels of unpaid overtime in a job that is extremely stressful. The teaching profession also has a high attrition rate of approximately 40 percent annually. Some areas and specializations are facing significant teacher shortages. Even worse, education is not a major attracting the best qualified applicants, with prospective education majors scoring in the bottom half of standardized tests such as the SAT and ACT compared to students interested in other majors.
What this means is that tenure, which offers a form of job security, is one of the few things that might attract and retain highly qualified applicants to the profession. It is also important to ensure that teachers can grade fairly, rather than feeling compelled to give students higher grades or teach exclusively to standardized tests to hold on to their jobs.
On the negative side, tenure will protect weaker teachers from being fired.