I will sit on the fence.
I agree it is on the face of it a good idea, and would not be opposed to it, just as I believe licencing, and training should be mandatory, and it would be a way to ensure that owners have lifejackets, flares and a seaworthy boat, but it then leads on to enforcment.
People would get away with not having it, You could make it a requirement for insurance, but then people would not insure the boats. Then who will do the inspection, I can see it would be a nice little earner for some, and would be open to abuse by others. To make it workable you would need a similar system the the current MOT for cars, and computerised records, and sanctions to the testing station if there were boats being passed that were not fit. A beaurocratic nightmare. It would be easier to insist everyone is trained and insured!
You could get Harbour Masters to try to enforce it at the slips, or before someone uses a mooring, but then smaller craft will launch off beaches, and larger ones will try to beach launch in order to avoid it. I have no doubt it works in other countries, and it might be worth looking at, but at the moment I can not see a way that it could work with our current rules (or lack of!)
The majority of Rib Netters I am sure would probably pass a rudimentary MOT, I guess a bit like the one the RNLI runs,used to be called Boat check was it?
Talking of which, I must remember to book one next time I am home!