i love how you've explained the rationale and game theory at play.
I've seen exactly what you predict in the official server i played before coming to HeRO - people breaking off in all directions for personal gain. What it means tho, is that all the noobs will steal whichever castle they can get their grubby hands on while the bigger guilds with god items in their sights will focus on a select few castles (of course this i assuming de-randomized chests). So what would happen is the "Real" action will be much more intense than it is now because the "big" guilds would probably be whittled down to 6-20 people (or at least 1 "good" 12-man party) so those "select" castles would experience intense fighting between the 2 or 3 "big" guilds while all the other noobs will go to the other castles as a solo sinX and experience their own intense battles between themselves
