A lot of users make very valuable comments which are often overlooked for various reasons. I was hoping this would give them some deserved attention. Also, having spent a bit of time looking at various users' comment streams, I've found myself having a better understanding of them — makes for a stronger HN community. And, finally, being able to make decent predictions is a valuable skill for any entrepreneur...
Hope that helps explain why I believe this is beneficial and not just fun.
I agree with the sentiment but not with how you're proposing to implement it.
You say "overlooked for various reasons". Let's work on those reasons not try to patch a system. Why should me being able to guess (not "predict" as you say) the value of a dinner translate into retrospective karma?
Also, what is karma, really? What is it really measuring?
Hope that helps explain why I believe this is beneficial and not just fun.