I should have clarified, I mean that it is my personal desire to have a permanently negative score attached to and displayed by my user account.
Jokes aside, this doesn't seem all that different from the "likes" system already in place. Even without such a system I've never had a problem in various forums finding out who was full of it and who wasn't as that usually becomes apparent with some browsing and through other users pointing out and correcting the dumbest things that are said by one user or another.
A good question to ask would be, is there a sufficient need for such a system to warrant the time that it would take to create, balance, and maintain it? Loops also pointed out that even if the system works perfectly on the first go, there may be both predictable and unpredictable consequences that are undesirable. See reddit's Karma system for a good example of this.
What situations have you experienced on this forum where you were unable to distinguish bad advice from good advice through context, and instead felt that such a reputation system was necessary?