Both knighthood and Kingsguard are important if we want to recreate the atmosphere.
Knighthood and its related things (tournaments, jousts, quests, oaths, courtly love) are necessary not only for flavour events and RP, but also for intrigue and plots. Heck, the whole Robert's Rebellion started because Rhaegar loved Lyanna and ran away with her. Or should we mention Jaime Lannister? The fact that he's a knight of the kingsguard prevents him from becoming the heir of the msot wealthy house of Westeros. Plus, as I said, it has enormous RP value.
As for the Kingsguard, being the fact that:
A) It's a great honour to be part of the Kingsguard, and it is an important factor in politics, both for the prestige of the position and the fact that a knight of the kingsguard is, by all means, hostage of the king;
B) They are close to the King (not some random obscure one-province lord, but the King himself!), which means that they can act as spies / advisors / assassins / betrayers other than mere protectors. They may even become Hand of the King.
We are not talking about nobodies standing idle and doing nothing. They come, most of the time, from important families, and can play, potentially, a huge role.
And anyway... RP, people! I don't know about you, but I want to be able to play 80 years in the game after the starting date and still have fun and a "realistic" Westeros-like environment. That's why we need flavour, and knighthood is one of the most characterizing features of the setting. It can't be left outside the mod.
I'm all for adding knighthood and kingsguard to the mod.