What's to stop me giving AWOL 100 karma by just clicking his Karma button 100 times? If I can, surely it is a useless measure, because it can be abused. If I can't, it must store somewhere that my user name (or my IP address) gave someone Karma. So, that's extra load. And extra load to check, for every poster, whether they can increase the Karma of every other poster.
And once I give Karma to one person, how long until I can give it again? An hour? A day? A second? More computations.
I just don't think it measures anything useful. Someone here may be an expert on something obscure (eg. UDP packets) and post occasionally, when they can say something useful. But when they post they give good advice. They might have low Karma. Others may post a lot but just ramble, but have a few friends that boost their Karma, like a mutual-admiration society.
For me the post count is a useful guide. Someone with lots of posts has, at least, hung around the forum long enough to hopefully know something about the Arduino, or at least where to find things out, or where another thread might be that would help someone.
For people with few posts, you can still respectfully read what they have to say, and if they seem to know what they are talking about, follow their advice.
So I don't see what Karma adds to all of that.