Sometimes, I do have time to read all the comments and evaluate for myself which are good and bad. That's great.

Other times, I don't have that much time and simply want to glean from the best comments on a post.

And then there are the posts in subject areas that are outside or on the edge of my expertise, where I have no business evaluating the comments. I can still benefit from reading them though, and the karma helps me give weight to the good ones and disregard the bad.