It doesn't really, not if you're in the right position. It even does a great job of keep rain out of your eyes when it's really chucking it down. Some pros (including Lance) use peaks when the weathers awful, despite the rule violation.
Well it obscures visibility for me - i usually wear a cycling cap under my helmet when the weather is awful and when on the drops i need to compensate by moving my head up to see the road ahead rather than just look up with my eyes. End up getting a stiff neck, which i dont otherwise get.
