It doesn't really matter what the purpose of copyright was in the first place. History is interesting, but it doesn't determine our understanding in the present of what is moral and what is not moral.

Also, copyright is world-wide, so what it says in the US Constitution isn't all that interesting to anyone not in the US.

And when I say "irrelevant", I mean it is irrelevant to public debate. The copyright industry's side of the debate about copyright enforcement is mostly the following, based on what I called the "moral model":

* We created something * We own what we created * Someone is stealing it

This message can just be repeated over and over and over again. Someone's report that internet piracy might not reduce cinema takings completely fails to address the above moral claim.

So the report can be ignored, and the moral claim can be repeated again. And again.