Given the objective evidence that this is not reliable as an interrogation technique, why was it done?
I find it difficult to believe that US military brass are ignorant or stupid. These people spend their lives studying strategy, intelligence, etc. When educated and intelligent experts suggest or do incredibly irrational things, I often wonder if there isn't some other reason that is not being publicly stated or discussed. In other words, do these irrational things become rational when viewed from a different perspective?
A somewhat cynical answer I've encountered is: it was done to deliberately foment anti-American sentiment based on a "fight them over there, not here" and/or "shake the tree" theory of operation.
A significantly more cynical answer I've encountered: the US defense establishment is an enormous make-work program and requires a constant supply of enemies to justify its budgets. By this reasoning ISIS would be an example of a successful result of this program -- a long-term enemy against which we can fight a low-grade war for a decade or more.
Of course I suppose I may be wrong. My assumption here is that educated and intelligent experts will behave rationally. This assumption actually underlies quite a bit of so-called "conspiracy theory" -- the reasoning process goes "elite leaders of business and government act like idiots, but they're not idiots, so therefore there must be a big nefarious conspiracy or hidden agenda!"
But maybe they are idiots -- or at least are behaving very irrationally due to emotional or ideological drives or bad information. As the saying goes: conspiracy theory is actually optimistic. It assumes that human beings are rational, resourceful, organized, and in control of the situation. The other possibility is significantly more pessimistic: the world is run by delusional ideologues and "peter principle" apparatchiks who really are that foolish.