Comment_karma is a remnant.

To my knowledge it has never been used, not even by b2. So it's not so much a remnant as a "there if you want to use it" thing.

We've discussed two possible uses for it over the years. One was using it for an anti-comment-spam system where multiple plugins could bump the karma up or down based on different factors, and the end result could be stored there. This was prior to the concept of storing "spam" comments in the database. Then Akismet came on the scene, and it worked better than open source rulesets, so the idea of a hoard of anti-spam plugins has faded.

The other one way Digg or Slashdot style comment ratings (not for core, but for plugins to use). That seems to be more or less the way True/Slant is using it, albeit with only two levels and editorial control of the ratings.