I know several users that are deleting their posts after a certain period of time for privacy reasons. I guess, most of them know that their posts are also tracked via tools like Ninjastic or BPIP, but they don't want the posts to be found directly via Bitcointalk.

I think that is a fair concern. Not everyone wants their posts to be found via Google at first glance even years later.

I do not believe that the post/merit ratio is the decisive point here.