>I think the biggest reason why such a feature would be a bad idea for an open image board is that it opens up an avenue for some serious mischief.

Could you specify?

I also disagree with the "only for pr0n" stance. Examples where someone wanted to post a series of non-pr0n-related pictures, yet they had to do so one at a time, are not hard to find:
http://kao.wakachan.net/r/res/631.html