The way ad serving is set up on the site is that you see ads based on your account status. If you are a member, you should only see an ad at the very top and very bottom of the page (on the forums). If you are a non-member (or logged out) you will see one ad per post.
In this way, it is a cheap incentive to create an account

Also non-members pay for more of the account hosting than the members do.