Whether you rule a guild, a temple, sources, or provinces, there are three ways to play Birthright. Players can adventure exclusively (and adventures can be realm oriented or not, players can focus on the wargame aspect of the game, aquiring GB, RP, and spending time (both IC and OOC) on the realm as the instrument of play, and of course you can combine them into a hybrid style of play.
When considering characters, the D&D game itself is designed for the adventure style of play. The realm part of the game was all new, and characters were not designed to win at this level of play. They are designed to do something else. A good realm character would be good at doing realm things, getting more GB, or RP, or really holding on to the loyalty of his people, planning and implementing realm actions, and commanding units in warfare. Given the way classes are designed in D&D it would make sense that a character would be really good at one of these and decent and another one or two, and weak at another two or three.
I know that people who play the adventure style of play see no use for these kinds of characters. But for those who either play Birthright like the old computer game, or like some PBeM’s in which play is based on the realm, not the adventure, there is the question of the character who is good at non-adventuring things.
Its also the case that many people just don’t like to see characters who are bad at what they are doing just because no one has devised a class for that activity. In most games, any activity outside of adventuring, is poorly or just not represented by class skills or abilities. But, in fact its just as easy to suppose that the commoner class isn’t someone who is bad at everything, he’s someone who is good at non-game things, like being a good parent. But in the BR setting, some of these activities make sense. A financier, whether a guilder, templar, or province ruler, someone who can really get the GB to come in makes sense, especially if play revolves around aquiring and allocating GB rather than the dungeon crawl.
Perhaps rather than looking at guilder and noble (which isn’t a really good governing class, but just a noble fighter), I suggest we look at functions pf the realm experience, and we should get natural classes based on these things. An Int based financier, who brings in the money, and may have secondary benefits in organing realm actions. A Wis based figure who is deeply connected with their realm. Not only do they get better regency collection, but they are more aware of what is going on in their realm. Charisma based beloved ruler, who has bonuses to increase and avoid decreases to their loyalty, and may get some minor benefits to RP collection as well.