If it were my game, I'd probably let people finesse scimitars since I don't think it would be particularly game-breaking. After all, a character can get exactly the same thing with a rapier, except for the change of damage type. It's true that slashing is probably superior to piercing for the D&D game, but I'd probably allow it anyway since I don't see it as being very extreme.
As far as realism goes, I'd say it's probably true that a person could learn to use a one-handed, curved, single-edged cutting sword in the way Weapon Finesse intends, so sure. I've seen people use Chinese broadswords (called dao) in very much the way you describe, so it doesn't phase me much.
-S