I've said for years that a Courtier/Noble PC Base class should exist.
NPC Warriors have the PC Fighter Class as a PC version, Adepts have Clerics, and Commoners have anything else. Aristocrats have, well, nothing.
However, the Aristocrat class has no PC analog, Bard is good as a general socialite, but Aristocrats are parts of the social fabric and leadership of a people, and thus definitely a "lawful" class, while Bards aren't even allowed to be Lawful. Paladin could be seen as a "noble warrior", but it's got a lot of religious and behavioral (and that stupid v3.5 summonable PokeMount, "Horse, I choose you!"). The closest thing there has been to a "Noble" class in D&D core rules was the 1e Cavalier.
I think the lack of a class for noble warriors and noble adventurers is just a legacy of D&D's roots as a wargame, and the focus on miniatures combat and combat-oriented games. A Noble could be a very powerful, very useful character in a game focused more on roleplaying and politics than "Kick in the door and kill the monsters" gaming.
In my own games, I use an adaptation of the Star Wars Revised and Wheel of Time noble classes (the biggest balance issue was how big to make the "Resource Access" class feature). I'd post and share it but I can't since it's based on non-OGC material. The Dragonlance setting has a Noble class (I'd use it, but I don't have a copy of the DLCS, I've only looked at it in bookstores), and it's pretty good, but I would rather see it in the core.
One DM I know lets PC's take Aristocrat at 1st level, as the only way to play a PC noble, considering the loss of game-mechanic power from taking an NPC class at 1st level (but they can multiclass to PC classes and PrC's freely) to be the cost of the social and political power that comes with being a nobility.