You're right, the total registered members isn't necessarily a good indication of how many people actually use the site on a regular basis. It's certainly true that members have registered, and then never been seen since - and dsmodi is also quite right there's nothing stopping someone reregistering under a different username AND email address.
As for unsubscribing we could easily add a facility to allow members to unsubscribe themselves, but that would leave the site prone to 13 yr-old hack3r deleting everyone! If members do wish to unregister and this has happened in the past, a quick email to me soons gets them deleted off the system, and their posts set to 'guest'.