Fenders will help keep road spray from getting on you and the rest of your bike. They won't really help keeping a skirt out of your spokes. Since I'm a guy I can't speak from personal experience just from what I've observed. I see women with shorter dresses and skirts ride bikes without skirt guards all the time. My guess is that trying it with a longer skirt is asking for trouble. I don't see how a skirt guard could hurt.