For most cabals, the people support the cause of the cabal, whether they're in there or not. Especially for fortress, if you're not someone who wants to defend the Maran whether you're a member or not, then why are you pledging?

For some cabals (like Empire) I can understand having a tit-for-tat attitude where you only lend your help because of what you get in return. But what kind of role exists for Fortress where you would say "Well, since I'm not a squire or scribe I don't care if the orb gets taken?" The "no skin off my nose" is really only the province of selfish evil-aligns. Everyone else should care about their cause enough to want to come regardless of whether they're getting the sweet cabal powers or not.