One purpose of (significantly) negative (total) Karma is to indicate when a user should be kicked out.

I disagree, total karma should never be the sole indicator of whether a user should be kicked out. If a user is being obnoxious enough that kicking her out becomes a possibility, this will be quite evident from a cursory look at her post and comment log. I'm thinking of a LW user who would make a perfect example of this, except that I just can't recall his or her name. Monkey(something), IIRC.

Total karma does have some useful technical effects, such as (AIUI) disabling downvotes and rate-limiting posts and comments.