The whole point of the MOT is to ensure a modicum of road worthiness, to exempt any vehicles is wrong. They all use the roads even if not often. Just because they are old doesn't mean the owners will maintain them properly. Do you really want to be clobbered by some old banger who's brakes fail !!!!
Of course we would rightly say that one dangerous car is one too many.. But how big is the risk when I haven't seen anything classic on the road for ages. Where are they all I wonder? If in private garages because they are worth too much to use then they are no risk at all.