A valid MOT certificate is important to drive the car home also, but actually is not proof that the what you are driving is road legal. In theory it is just a snap shot in time, even if it has been done a few days ago, tyres, wiper blades etc all could have been changed or worse. So a check of the vehicle should still take place. The UK has quite a thorough MOT test compared to other European countries, so in reality on a mall amount of vehicles are affected, modern manufacturing processes also means a most vehicles can have a longer road life, should its value be worth it.