Well if the argument is "because it's expensive and, unlike insurance and road fund license, doesn't make the government money" then really it just goes to further prove that the UK motorist is a cash cow for the government.
By law we must get our vehicles MOT'd to ensure that they are road-worthy. Yet VOSA make a point of saying that just because your car passes the MOT and you have a pass certificate, it is not a guarantee of safety or road-worthiness.
So then, what is it exactly? Other than a way of getting motorists to fork over more money and to give vehicle repair companies an additional stream of revenue, wherein the government will happily take 20%. Smells like money earnt to me - and they won't send you a 10p text message for the privilege? Thanks! We're all in it together though - remember that. It makes it all better.