Believe or not karma/activity metrics encourage participation/activity, that's one of the reasons for the success of some social web services, they have a metric system that encourage some kind of competition, there's a couple of interesting studies that prove that.

In my opinion the gains of a karma system are bigger than the losses, so yes, we should have some similar concept.

PS- For those interested in these topics about social interaction in the web, like me, here's a cool presentation: http://www.slideshare.net/stephenpa/...dea-09-version