I'm in favor of the lower limit. There's no reason that a spammer would be able to get even 1 vote, so that should take care of the problem by itself. Getting 5 karma in the discussion section is easier than the main page, as there are more posts that a newbie can contribute to. Plus, we don't want to drive away people who are frustrated that they aren't able to comment on discussion posts.
A spammer could very straightforwardly get as many votes as they wanted by creating sufficient accounts to that end. This would be exactly as effective as a dialog on the submit page that says "Put the letter after Q in the alphabet in the dialog to submit": ie it will stop any bot that is not tuned to this site, but allow through any that is.
Admins have been doing a decent, timely job taking down the spam that comes up in the Discussion section. But it is an eyesore for any period of time and there seems to be more and more of it. And there is an easy solution: a small karma requirement for discussion section posts. I think 5 would about right. A reasonable, literate person can get 5 karma pretty easily. "Hi, I'm new" usually does it. That plus a half-way insightful comment about something almost definitely will. This would screen out the spammers. As for the occasional genuine user that posts in discussion before commenting at all, I don't know how many there have been but my sense is that delaying them from posting until they can get five upvotes is almost certainly a good thing.
Thoughts? Or is changing this actually a difficult task that requires rewriting the site's code and that's why it hasn't been done already?