Jeff's got a great post on the role practice experience ought to play in faculty hiring, of which I will (perhaps all too predictably) just excerpt the snippet about yours truly:
I do think there is something to requiring an aspiring full-time tenure track academic in a university or quasi-university setting to signal (to Brian Galle's point) what I would call a tolerance for, if not a commitment to, in the absence of a better term, "the life of the mind." (Put aside my belief that there are whole areas of practice now the exclusive domain of lawyers and law schools that could be taught and practiced without a three year traditional law degree, and the university model need not apply there any more than it does for barbers or chefs.) The person who comes to mind is my friend Steve Bainbridge, who quite publicly proclaims (often and loudly in a metaphoric way) his impatience with both "law and ..." and empirical legal studies and his preference to focus on the law. Nevertheless, I don't think you ever pick up from Steve a disdain for intellectual pursuit or think of him as anything other than a university professor. (You can pick up a lot of other clever disdain from Steve - that's why we who disagree with him so much still love him - but not disdain for thinking!)
Here's my take: A good law professor needs some practice experience, if only just to get acculturated to the profession into which s/he will be directing students for the rest of his/her professional life. More is probably better, although I think my shortish stint in practice has not been an obstacle to staying current or to being engaged with practitioners and the bench. It's mostly a matter of attitude, I think.
A good law professor also needs a commitment to the life of the mind. A commitment to doing serious but engaged scholarship of the sort that the bench and bar can't do but will find useful. A commitment to writing often and well. A commitment to keeping up to date with developments in your field OF LAW (not your law and fill in the blank field). A commitment to going where the evidence takes you. A commitment to not whoring yourself out as an expert witness or lobbyist. Mostly, a commitment to revel in the intellectual freedom this job allows. The law is an endlessly fascinating subject. A good law professor will commit to mastering some part of it at a level no judge or practitioner would ever have time to do.