You get comment karma every time you make a comment.
No, that's not how comment karma works. In order to get comment karma, your comments have to be upvoted by others, you don't get it by simply commenting. So those that write nonsense get nothing at best, and they can even be downvoted and loose karma.
Karma is used as a relative measure of activity which is why subreddits have metrics of minimum karma to participate. Bitcointalk already has an activity metric. I don't see a point to merit at all other than bragging rights.
You can be very active shitposter on reddit, and you probably won't receive much karma, therefore preventing you from joining most popular subreddits. It is similar with bitcointalk, you can be very active shitposter here and you will gain activity, but you will never reach higher ranks and some benefits that it offers.
So i would say that both reddit karma and bitcointalk merit have its purpose.