@Richman Percy Jackson and crew were lucky they never actually really
needed to use their weapons against mundane humans, though. If you've ever read Warbreaker by Brandon Sanderson, you'd have an idea of why narrowing the sword's usefulness to such a degree can be a bad idea. If you're unfamiliar with it, there's a sword in it called Nightblood, who is sentient and was created with the sole directive of destroying evil. But that brings up the question: what is evil? Who decides what's good and what's evil? For a sword that doesn't understand human morality
or have a concept of good and evil, "evil" is basically anyone and everyone, but especially those who might be tempted to pick up Nightblood for any reason.
Limiting the sword to cutting "sin" is almost worse, since sin can be any number of things in any number of religions just here in the real world. Toss in the morals of a fantasy world where pretty much anything goes and there's no telling what the sword could or could not cut.
@guih34 Not all styles of swords have them; it all depends on the style of swordsmanship it's used with. The Russian shashka, for instance, lacks any kind of guard, but that doesn't hinder it as it's used in a fashion that beats an incoming blow aside rather than actually allow it to make solid contact for a bind, the way you would with, say, Talhoffer. Many swords of the Iron Age and earlier lacked the broader guards of their later counterparts because they were used almost exclusively with shields, so the guard didn't have much purpose.
Plus, a guard is really only useful if you actually plan on getting into a sword fight; I kinda think he'll just slice through anything that gets in his way, whether it's a sword or a person.