Well, there's also signal-to-noise ratios. There are some people who have high karma by submitting dozens and dozens of sites knowing that one or two has got to stick. To me that degrades the quality of the site as much as spam does. I guess your solution might solve that in a round-about way as well.
I would abstain from complicating the user experience, though. In my experience, posting a topic has much more potential in gaining points than writing *thoughtful* comments. A good strategy to rise above that ranks is to post a bunch of interesting links; upon achieving high points, the user can then take a defensive stance by defending his or her opinions through posting comments instead.
The point is, from a user's perspective, there is more to gain from posting topics than comments.
That is certainly the status quo; I would ask, however, whether it is desirable. I think the system should provide more incentive for thoughtful comments, or at least some sort of disincentive for shotgun-style submissions. It is difficult to build karma by posting lots of comments; you have to put the effort into generating good content to get karma. The same should ideally be true for submissions; the difficulty, of course, is that since the content of submissions is generated by someone other than the submitter, simply posting lots of them /will/ eventually result in a karma-boosting hit.
We had a similar problem on our social network where users abstained from giving karma to thoughtful comments. Consequently, we coded an algo to add "+0.1 karma" to every user that gave another person "+1 karma". That was enough to take care of the problem.
