The only problem with a forum activity reward system is that it's extremely easy to abuse. Even someone with near zero scripting knowledge can create a script that browses the forum for them. If we require people to post replies/threads then I already know it will be mostly spam.
Also if someone is on the forum then we can't see what they are actually doing. For all we know they have the tab open 24/7 and don't ever pay attention to it.