Having less fiefs means the owners can defend even higher. Kinda screws the lone wolves. Back in the olde days I tried threaten individual holders with war, but they and i knew I wasn't gonna waste armies fighting a neutral when I was at war with pretty much other faction out there. Plus it makes trading harder. While I suppose the idea was to make people fight more, in fact it will make it harder for people to fight at all. If I choose not to be part of a faction, I cannot trade and therefore I cannot make money therefore I cannot equip my troops therefore I cannot go to war.
My opinion is if you wanted to generate more interest in this game, you should have added more fiefs. With more fiefs, small tiny factions might go to war against each other, but as it is a 3 member state will not try fight anyone but just sit and try hold their fief.
I really think that instead of crime you just limit the garrison to 50% of your population then there is hope. This bloody game of attrition is boring. If garrisons are limited, then maybe we'll see more field battles as people counter strike and try prevent a siege in the first place. And maybe that will add a little bit of strategy in this strategy-less game of strategus.