i think it makes her stand out. when people learn they think that the bender needs to see what they are doing to ensure it is right. Like when you fire bend if you kick and you can't see how do you know you created a fireball and not just a blank kick?

When you water bend how do you know the water is moving the way you want it to move, unless your hearing is incredible and you can pinpoint exactly.
Earth bending is probably the only way that allows blind people to see. If anyone else was blind in the avatar world they wouldn't be a bender or would be incredibly gifted.

Also allowing blindness gives much comedy relief, which i must say Sokka does well as it is. Toph uses a mix of seriousness and humor at the same time. Example would be lake laogai. She acts serious and mad and slaps some glue on a wall and smacks a poster on it, then goes and acts humorous as she asks if it's upside down which was actually backwards.

Not sure how that helps, but thats my 5 cents for you, (hey thats a nickel!)

Yeah, I agree with Hieu, sorry, Takano. (still getting used to that...). Toph's joke in Lake Laogai when she said that Sokka's drawing looked great was one in two blind jokes. The first was in The Library, when they were talking about books. But in water bending, couldn't you be able to detect the water currents under the earth? Or something like that.