Every guest and every user have its own session (Created/updated etc.).
While users logged in sessions is normal thing, guest sessions are not.
I mean, what is the reason to every guest have its own session? They are all the same for guests (except session_id) and if all forums are accessible for only logged in users then every guest could use one pre made defautl session.
Only functionality that would be lost is guest counter in forum online statistics (index page). It could be calculated statistically (about 8% of logged in users are guests), so thats not really a problem.
If forum have thousands online users at once, it would speed up forum especially when it is on MyISAM tables (less insert/update).
Am i missed some another functionality or maybe security?
I think topic is for phpbb3 and phpbb2 at once.