I would say no. This is why. This site is moderated sufficiently, it is not needed. Basically if you don't have a good reason, keep it simple. And such items tend to turn these forums into popularity contests with people all trying to please rather than people really being honest. That may make you popular, but it really does not help anyone in the long run. It is generally self evident who is helpful. Best help you often get is in the form of something you really do not want to hear, and one probably won't be clicking a "thank" button in such a case (although they may want to thank the person later when they realize that they were just telling them the truth, as hard as that was to swallow at the time).
A considerate and well reflected answer to a question can be seen as positive, even though it doesn't meet everybody's "needs". If you explain thouroughly why the truth is what it is, and try to comfort or even be somewhat empathetic, the person you answer will thank you, for showing that you don't write something "negative" just to be mean.
I like this concept a lot. Yes it may be about popularity, but if a "Karma-counter" is what it takes to make this a better forum, then why ignore the opportunity? Instead of having MVC polls based on subjectivity from users who happen to be active at the time of the poll, why not have an infallible system that actually shows how helpful you are statistically? Personally, as I've said, I don't see anything negative about this idea. Great thinking, Vaidya.