Well, there are some spam diaries... Some people are even known for having nothing but spam in their forum *cough* Francis *cough* And then some people are even known for inactivity, but let's get to the point
I feel that if a member has those kind of spam games, then they should PM an admin and request that their forum won't add to post count. Other member forums (like mine... -_- and don't get started about the inactivity thing) barely get any traffic, and the posts they do have in them are full of content. So not every member forum/diary needs to have the post count increment disabled... But, could always use more opinions