Though negative karma has it's place, probably, i think it would be abused far more than positive karma has or too much.
-It is safe to say that not everyone who deems your post as useful will grant you positive karma.
-It is also safe to say that every person you annoy with your post will
give you negative karma.
Adding negative karma onto the list would just lead to seeing who has the negative number closest to zero...I for one would be in triple digits I'm sure. I'm not always as civil as I preach to be.