The reason is a fundamental tradeoff -- we've accepted a bargain to deny ourselves some fundamental property rights temporarily under the theory that, in the long run, we'll get more innovation and content and further the "progress of science and useful arts", as the US constitution puts it.
If we're not making that progress, then it no longer becomes a tradeoff, and the laws are simply breaking property rights for no reason.
So, yes, if you care at all about property rights there's a very good reason to believe in limiting copyrights.