I simply don't agree, because it allows users to censor certain posts, as it happens with YouTube comments. Anything that allows a user rating other user, directly and indirectly, is a bad idea. That is why some SMF administrators don't implement the karma feature.
Of course, besides the karma feature there is always the temptation to disable a user with a bad karma, ultimately enabling revenge and user "bans" by users. If the idea was implemented, most posts would have a negative rating. You can mark my words, since in SMF forums having the karma feature, most users have negative karma (below -30) and a few have a positive karma (above +2).