I think that the fundamental change is the hidden(?) fact that it is no longer the vehicle that is being taxed but, the owner.
Time was, you could buy a car with 3 months tax on it and not have to worry about it's legality because the outstanding VED 'came with the vehicle' (unless it was 'cashed in' by the previous owner). Now, you have to tax it immediately because the outstanding VED is not transferred with ownership.
(I wonder if anyone will be booked and 'criminalised' by the feds whilst driving their new purchase home?)
OK, so the (whole month(s)) refund is paid to the original owner and the residue (days) disappears(?) into the coffers which is, imho, significant but, not a vast amount for the treasury.
This is no different to the 'old' refund practice except that the middle men (garages etc.) no longer get to cash in outstanding VED on trade-in's as a 'hidden' profit.
Personally, I think that the DVLA should have left things as they were as the new scheme seems more that a little 'cock-eyed'. What's the betting that they mess up?