Dont forget that an MOT is only a check for the road worthiness of the vehicle on that particular day. Wether the car is 4 or 40 years old, the onus is still on you to maintain it in a safe and road worthy condition i.e. bald tyres and dodgy brakes will still get you in trouble with the law and your insurance company, even if the MOT was passed only a few weeks previous! The issue is far more relevant IMO for corrosion related issues that may go unnoticed and not so much the less than ideal mechanical condition. How often do you or someone else get to have a proper good look underneath the vehicle without it being at a garage, either up on a lift or over a pit?!