I did think a lot about your critique of the Karma system. The way you see it, it makes a lot of sense. But I will bring you the point of view I just got there.
Let's say that someday, somebody do a nice trick tip thread in the forums. Everybody would rate him up. Then the next day, action/reaction, he would see his Karma increased by a few points. But the next day this guys is a total douchebag about how skiing is more skiing when you do it in a certain way. So, with the actual system, at that exact moment you are going to rate his Karma. You would have a feedback of how you rated him last time. Then you would have the choice, to retract you previous rating and bring him to 0 or just re-think why you did rated him in the past and maybe, now, you would rate him down but not to 0.
Pretty much, it means more power to the actual vote you do "right now". And the Karma can fluctuate up and down depending on the latest action you just did on this site. Instead of having your vote piled to the 10 others votes you previously did to him. Which is piled with the other 10 votes of 200 persons who did rated him. It means more power to "now", when you read the thread. Because it's a lot less diluted.