I find it useful because when browsing a post, you have the most upvoted or commented on at the top. I don't see how it increases top-level comment competition unless commenters are trying to get their comment up to the top? I think if you hide the karma, and that would also including hiding how many upvotes/downvotes your comment got, then you can't really track the "competition" and makes it much less competitive (and potentially addictive).

(I do see your point about not being able to keep your place, though I don't systematically go through all comments in a post -- or very rarely -- more just quickly browse for things that catch my attention, interesting info, etc.)