I agree with Alice on this one. There needs to be some kind of balance. Teachers, especially those of college level, need to keep up with new advances in their field at the very least. Contributing to their field wouldn't be a bad thing either. However, they need to be available to their students too. Depending on the area of study, only being there for classes a few times a week may not be enough. I only studied English in college, but still, it was nice that my teachers had office hours, and were quite willing to help us with assignments during those hours.
And there does need to be some protection for these teachers. If they have ideologies or theories that go against the mainstream, they shouldn't be afraid to teach it. However, without tenure, they could be fired if enough people don't like it. That's censorship, and not a good thing.
However, tenure in K-12 probably doesn't serve much purpose. They teach what the school district tells them too anyway.