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While I do not know that those people turn out to be more productive than others and I can think of examples when they have not, it seems to me that the more crucial issue for a mid or lower level law school arises with respect to the next level of hiring. The choice for these schools is between second tier elite school graduates and the top graduates from non elite schools.
I do not know what every school does when faced with this choice but my impression is that a fair number of them go for the second tier elite school graduate. What I do know is that this bias cannot be based on any empirical comparison of the productivity of second tier elites and top tier non elites. No, it's purely self-referential hiring because God only know what would happen if the top ranked non elites were hired and out-shined the elite. We couldn't have that, now could we?