Tenured researchers shouldn't be teaching, IMO. They can be doing research full-time, and leave teaching to those who care to teach. I, personally, dislike research, but I would love to become a CompSci professor and teach the next generation of software developers and technologists. The problem is, I do NOT want to do research. Give me a Master's in CS with some sort of "teaching extension" on it and I'll go teach and the researchers can go do important work researching!
Edit: for the upper division courses, researchers certainly should have some involvement, I agree with several of you on that point!