In D20 it's lousy, you have to have a Weapon Focus AND a Finesse for each wapon, then it only let's you use your DEX instead of STR for attack rolls. Since there is no DR inherent to armour, there is no other benefit.
Actually, in 3.5 they changed it a bit so you only have to take Weapon Finesse once and it applies to all light weapons (plus rapier, and a few others, I think). Makes it a bit better at least, I think its a pretty good feat for most Rogues.
Weapon Finesse in Conan also applies to such weapons (those which don't do massive damage like 2-handed swords, but which can be maneouverable).
Just to clarify one little point (I've mentioned this in a dozen threads, not trying to get anyone's goat, just talking about Rapiers as I teach their use on a weekly basis): rapiers are actually heavier, longer-bladed, wider weapons that were much sharper-edged and point than the DnD game stats represent. Rapiers were slashing weapons; transition-rapiers were cut and thrusting weapons, and smallswords (what most people think of as dueling swords, from the late 17th-19th centuries) are thrusting weapons. The foil was invented to help train in the practise of smallsword fencing in the early 1800s. It was never used for combat, though it is the Olympic weapon we all know nowadays.
a transition rapier
Smallsword duel choreography
Okay, I'm done with my post, just wanted to clarify, thanks for bearing with me!
Rapiers don't really belong in the Hyborian World, but arming swords, short swords, and similar categorized weapons all can be used with weapon finesse. :twisted: