I think there are much better ways to determine how well a staff member is doing other then depending on the number of posts. Quality beats quantity any day in my eyes and for a moderator, not all the tasks they do are visible. Moderators need to be online to deal with issues, delete posts/spam and resolve issues usually via PM. They can do all this without posting. If you have a post requirement, a moderator could come on for 5 minutes a day, post 5 welcome replies and ignore reports and issues needing moderator attention. Does that make this moderator better at their job just because they make 5 posts a day? No in my opinion.