the overall sentiment of your idea is great! it promotes people helping other people, and that is good for the community.

One problem is: if you implement extra points for posting on help forums, the moderators need to pay more attention to the quality of help and delete 'spam thread' posts as well. This also means people's bonus points may need to be taken away if they make fraudulent posts... then they want to appeal it all the time, etc. etc.. It sucks that it would need to be 'policed' like this, but if given the chance, I'm sure there's a few people who would try to abuse the system.