That's true, my advice is more in the lines of don't go to law school if your only intention is to become a professor, I guess. Sort of like, don't go to a law school if you're only going there to transfer out. Chances are slim.
That being said, I've also had professors who graduated lower ranked schools, so I guess it's all about working for it. At least one of my professors graduated Rutgers Newark, and if you can survive 3 years in Newark, you're pretty damn hardcore in my book.