First, NO it was NEVER BASED ON HOW USEFUL YOUR POST WAS -don't be starting rumors.
It is a simple way to recognise the posting contribution of a member, not on the post's merit or usefulness, but the simple fact that the member took the time to either post or reply to topics. There is no hidden agenda to that system - it is based on milestones in the number of posts made by any given member.
Colors may differ - that is member-status - member, mod, global mod, admin - but number of stars is simply post count.
There is a feature called KARMA which (although I believe in karma as a Life Rule) I wouldn't use it in the forum, it is a means for members to evaluated the positive or negative "Vibes" a post leaves - a sure fired way to TRASH someone you don't particularly don't like - we don't need that feature, thus it is turned off.
Hope that clears that up?