I've heard people argue against helmet lights for the same reason. I don't have a camera (yet, can't afford it), but I run a light on the top of my helmet. I can't envision the crash where I'm sliding along the ground on the top of my helmet, and in fact, studies show that a lot (most?) impacts to helmets are on the side. As long as your accessory is mounted on the top of the helmet, I doubt it'd have any negative effects in a crash.