Here's my take:
Most of my organic traffic is sent by search engines... mostly google. People search for a topic and click through to my site. Most of these 'guests' are looking for something specific and will never register or become a member. They will receive ads.
Registered members contribute content to the site... which ends up indexed by search engines... so they may use the site ad free. They grow the site and in the long run help bring in guest traffic and new members. I also believe that members that use the site regularly know where the ads are located and avoid them... maybe even block them... so why serve them ads at all? It also makes YOU, the admin, look like a hero by offering them a website advertisement free.
This model has worked great for me on a site with fewer then 10k members but makes hundreds of adsense dollars every month.
Good luck,
-JRW