I have come to favor tenure, when I used to think it a bad institution.
The reason is that it protects, and selects, for the most part.
It protects by giving people freedom to work on better projects, and to have wider choice in subjects.
It selects by choosing only those people who will continue to work hard AFTER tenure.
Now, many lazy people NOW got tenure years ago, when it was easier. But I watch the people I have seen get tenure in the past fifteen years, at top departments, and nearly EVERY ONE publishes more, the work is better, and they do university stuff like talk to prospectives and attend graduation.
This may not be true for economists, or other interplanetary travellers. But it has been surprising to me how often people work harder and better once the threat is removed. That leaves the real reason, protection from political or personal bias.
And, G. Mankiw nails the comparative static result. All departments would be ruled by powerful heads, and closely overseen by even more powerful deans. Icky-icky-oo.
Frank, one way to think about tenure within a free-market/classical-liberal mindset is to compare it with surgeons working at a hospital. On the one hand, if they're incompetent, the hospital has an interest in being able to get rid of them. But if they're actually very competent, but mavericks w.r.t. "the rules," or favor techniques or approaches which would irritate the hospital bosses but actually help patients, then it would be good if they _couldn't_ fire them. Academics works the same way. If Prof. A is incompetent or dishonest in objective and demonstrable ways, he or she should be dismissed, and indeed most institutions AFAIK have tenure-defeating provisions for things like not showing up for class or plagiarism or armed robbery. But if Prof. B pursues an unpopular or un-PC research agenda, it's actually better for the institution to have a built-in protection for that prof, rather than have the prof always looking over his shoulder at an angry mob. It's truth-seeking, not American Idol.
I agree with the comments above, but have another take on it as well: tenure is a non-cash form of compensation. If Universities were to get rid of tenure, the cost of retaining faculty would go up very quicky. Granting the "annuity" of tenure is a way of paying a faculty member without having to take a hit in the cash budget. There is, of course, a real economic cost to it, but it does not affect the current-period budget.
This is also why I think that the schools that have tried to abandon tenure wind up reinstating it: they simply cannot afford the higher costs the have to pay in the long run to retain faculty.
Here is an interesting question: how much money is tenure worth? That is, how much would one have to be compensated to agree to permanently give up tenure. I'm not talking about a case where say a faculty member leaves school A to go to B and gives up tenure - in those cases the person realizes that will, in all likelihood, get tenure at school B and so will leave for much less of an increase in wage than if they were really losing tenure for good. My question is how much would it take to get one to permanently give up the opportunity to earn tenure? Clearly it will depend upon both the school one is at and one's assessment of their future marketability, but my guess is that most faculty in econ or finance at major research schools would require a very substantial lump-sum payment to make such a change.