I lost several points karma today and it prompted me to search my previous posts to make sure I hadn't said anything dumb or offensive. It lets me know how I'm really behaving, as opposed to how I think I am. I care a lot about what people think of me, and with my activity levels I find it important to show I'm the type who can be a role model, as new posters will be looking up to me (Especially as my post count rises). It also helps prevent people from getting upset when someone else says something rude. They'd see that the rude person has a low karma, and it lets them know that the forum is aware of this person's behavior, and isn't tolerating it.
Low karma isn't too hard to recover from either, I spent quite awhile with a - karma when I first started. I've only seen one person with a karma below -5, and a group got together to raise it back to zero.
I just say we take it lightly, and have fun with it.