I have to say I have often thought this myself. While on the plus side bikes create no polution when they are being used as they rely on human energy, yet we have the argument that they use the roads like any other motorised vehicle (Car, Motorcycle, Van, Lorry etc.) yet they do not have to pay any tax for being able to do so.
Motor or not, if its on the road it SHOULD be taxed and insured. I appreciate that someone has said what about childrens bikes, and thats a very good question, not sure on that one myself.