You make good points and are probably correct that the amount of "police interest" might be low. However, police are very interested in motoring crime because the law is such that if a vehicle carrying your number plate commits a crime you are obliged to inform the police who was driving at that time or there is a penalty. This makes many motoring crimes very simple to detect and the motorist basically has to volunteer for the penalty or they will be subject to an even greater penalty, in terms of clear up rate it is a real win for the police and so they chase many motoring crimes with great enthusiasm.
Another angle on this is that I don't object to my car carrying a number plate, in fact some people are so attached to their number plates that they get personal ones that reflect their personality in some way. Why would cyclists object so strongly to number plates, licensing, and insurance when motorists generally embrace these things and appreciate their benefit? Putting a number plate on your bike is no great tragedy unless, of course, you are about to do something where the number plate could be provided to the police to identify you.
The other thing I will say is that I don't think the problem is tiny. It is many years since I saw a motorist do something that left me wishing I'd got the number so I could phone the police but when I walk around the town I frequently find myself wishing that the bike I've just observed had a number plate. One day recently I was nearly knocked down, by a cyclist, on a crossing, with the green man on display, as I crossed the road and the same day I watched a cyclist fly through a crossing (against a red light and with the green man showing for the pedestrians) passing between two elderly ladies who were chatting as they crossed and leaving them shocked but thankfully untouched. Most of us have also seen the cyclist who steams through the red light leaving motorists to take avoiding action etc. I would guestimate that, based on past experience, if bikes were to get number plates then I'd be reporting a cyclist to the police maybe 2 or 3 times per week. Unless, of course, the sudden loss of anonymity produced a dramatic change in behaviour. It would also be interesting to see if, after the introduction of number plates for bikes, there was any reduction in the number of cyclists involved in collisions. I would suggest that having their name and address on the back of their bikes in the shape of a number plate could be a big safety measure for many cyclists, indeed many of the victim cam cyclists might give up cycling altogether and move on to another hobby where they can be anonymously aggressive and get away with it.