The Government carried out a 'survey' about this, before it was implemented, and based on the 'yes' replies and a desire to 'help' the motorist, it became law. Their feeling was that 'classic' car owners were responsible people and could be trusted to keep their vehicle roadworthy. However, the FBHVC, which is the voluntary body that oversees the British old vehicle movement, does not agree with the decision, nor do most of the clubs and magazines, for the reasons mentioned elsewhere. Initially, pre 1960 cars were not even going to be allowed to have an MOT, but after pressure from the FBHVC, pre '60 vehicles can submit for a voluntary MOT, which is what most owners will do. I have owned many such vehicles and was competent enough to do my own maintenance, but I would still like my car to be independently checked once a year. Even if the car is professionally maintained, it is no guarantee that the work has been done properly or that everything has been checked. For members old enough to remember the Rover P4, from the 1950s and 60s (60, 75, 90, 100, etc), they had king-pins filled with EP90 oil, but when they they leaked, they were often filled with grease. I purchased a 1956 Rover 90 that had new king-pins fitted, but the 'garage' had filled them with grease, which I had to flush out and refill with oil. The problem with grease in this application, is that it dries out, becomes infective and promotes rapid wear of the king-pins. This is just one example where an owner might think his king-pins are OK, whereas a MOT would pick up a problem. Additionally, few DIY people have the use of a hoist, and with the best will in the world, many items cannot be easily checked on a jacked up car. The insurance implications of not having a test, don't bear thinking about.
Personally, I think testing should be applied to all vehicles, even those under three years old. Some owners do 50,000 miles PA, so in three years, that vehicle has done 150,000 miles and is probably on its third set of tyres. I have often pointed out a defect to a motorist and their response is usually, oh, I will get that done at the next service or MOT!