With a helmet, you need a pair of side mirrors even more.
Do you know how many times i have been through near death situations just because of some f*cktard on the road who doesn't know who's behind him? And thats especially the case in traffic areas, where bikers change lanes without knowing their rear tyre. Just yesterday, i avoided an accident 3 times (by the same biker) who was changing lanes, and bypass cars in a huge traffic ... and 3 times his rear tyre came in contact with my bike's front tyre.
Anyway ... If you use a helmet and a protection kit. It means that your brain is not dead and you have a 'sense' of following rules in you. But NOT using side mirrors is the worst thing a biker can do :S