Yes, most of us are capable of checking our bikes, and do this on a regular basis and we know if something is at fault and whether that fault is dangerous to ourselves and/or others or just 'iffy', but until ALL of us fall into this category then sooner or later, someone who isn't as knowledgeable or actually cares will have or cause an accident!
The exemption aplies to all Vehicles of Historic Interest (VHI) which is all our bikes that are already in the 'Historic' taxation class. It is for vehicles that have not been 'subject to substantial change' during the past thirty years. Although how will they police this, who knows????
On the other hand, most of our classics do very low mileages (if any) each year, so its unlikely the condition will have deteriorated to the extent of affecting safety. Having 3 bikes on the road, I doubt I will do the voluntary MOT on each bike every year, however I will do one as each bike has done say 5000 miles for peace of mind.