Then how do new users get started? They have to write comments? Most people don't want to do that.
Another idea is that posting one article a day could be free, but ti costs karma for each additional one posted (maybe even an increasing cost based on how many you post per day.)
Requiring that users acquire at lease one point of karma from posting comments before they can submit stories would also eliminate the random floods of spam submissions, since spammers aren't likely to either (a) post comments, or (b) have the comments they post get voted up.
Requiring that someone has SOME positive karma before posting is just a way of saying "yes, this person has shown some interest in participating in a reasonable manner".
And definitely put a karma requirement for polls.