Well, they do look a little lame on a fast bike, right? Also they're heavy, if you're concerned about weight. On low-spoke-count wheels, they can have a tendency to loosen up and slide around, making noise and generally causing distraction... If you're riding during the day, they aren't doing much good, and if you're riding at night, lights are mandatory anyway, so are the wheel reflectors really necessary, esp. given how bright good blinking tail-lights are, and the fact that you'll probably have additional blinkies/reflective clothing... (someone's going to disagree with this though).