How come we never saw Rosalie fight in Eclipse?

This is from an interview that Nikki Reed gave:

“That’s the process of moviemaking,” she says. “I also trained for seven weeks and didn’t end up in the battle sequences — that’s a very silly mistake that we made.”

A glitch in the lighting of her action scenes meant her work couldn’t be used in the movie’s climactic conflict between mga asong lobo and good and bad vampires.

cause they had problems with the cameras so they didn't use Rosalie's scenes during the fight

Because she was being ladylike or something but she will be fighting in breaking dawn i can tell you that much!