when the forums first went to SMF, the karma feature was left on. It was quickly proven to be pointless and turned off.
rating of posts however, do come with positives. when set up properly, they allow for those people that are active within the community easier ways to have their helpful replies stick out for others searching the forums.
but proper implementation would require a complete overhaul of the forums; one that will never happen, because there are no active admins with the time and resources to be able to do it. and also, as it was only agreed upon switching to SMF from the old forums before, it was because we could do so without losing any posts or having to start completely fresh, so that would also have to be met before we updated the place once again.
these forums could be far better than what they currently are. the code behind this version of SMF is dated, and there are far better packages out there that could be used. if i were in a position that i was allowed to make such decisions, you would all see far improvement with these forums, but it is not my decision on what code runs the forums or what features get in and what ones don't; I'm simply here to manage them.
that doesn't mean that new features such as this one can't be suggested and debated on however. the team does listen, and with enough positive feedback, changes do come about.