I will be very pleasantly surprised if the Tories/LibDem's re-instate the rolling tax exemption - even though they should as the current situation is a bit unfair. I can only wonder how many low value cars around the 25 year mark are maybe scrapped because the annual road tax costs so much? The volume of cars over 25 years must be so low that it would make negligible difference to national debt.
Also, I think the road tax system is a bit of a joke - being based on emissions of the vehicle it takes no account of the envrironmental true cost of manufacturing that vehicle. e.g. the other day I read an reputible article claiming that latest electric cars are actually dirtier than a small diesel car (because of the CO2 created by the electricity used to charge the thing).
The general theory that I tend towards is that the longer a car is on the road, regardless of how "dirty" it's emissions are, the greener that cars is. So, methinks current road tax system is generally a ploy to flog more new cars. Though, on a plus note, the advantage is that, over a long period period, even the old cars will be a lot greener.
Now I am arguing with myself and deviating. The point is we should have free tax on all cars over 25 years and that, once a car has been manufactured, the greenest thing to do is keep it on the road as long as reasonably possible.