I think the bigger problem is that people on bicycles don't always obey the laws which are in place to protect both them and motor vehicle drivers. The parent mentioned cyclists who run through traffic lights and similar infrastructure with wanton disregard for the situation (I have witnessed this many times myself). This creates a situation that is dangerous to everyone involved and is quite illegal. However, there is no recourse as there is no easy way to identify them as you would a motor vehicle as there are no plates or identifying items on the bicycle.
Ultimately I agree that adding plates is a bit extreme, but I think it's also disingenuous to say there is no problem and ignore them.
Edit: I would also like to note that I'm not commenting on motor vehicles disobeying laws in similar situations, they routinely do, however that does not take away from the fact that there is an issue here as well.