Good point, can't really argue with you there. The only problem is you'd have to have bikes with number plates and all paperwork like you do with a car, it would be more hassle than it's worth.
There has been some serious talks on this matter recently within our government, with so many cyclists being hit on the roads.
There is the idea at the moment, that cyclists will need tax(though will be £0), mot (In the same way as a car, breaks & tires) & insurance. I really hope they do, as some insurance companies can be arseholes when it comes to who's at fault when a cyclist is involved. It's ok for them to claim from you, but when they cause the damage, it's like getting blood out of a stone.