I think that the RAF needs a mix of DE and UAS blokes. As educational standards plummet (as evidenced by grade inflation and the proportion of those going on to Uni) I believe that if the RAF is to get people of the necessary calibre it will have to increase the proportion of University calibre entrants (who are now broadly equivalent to DE blokes of ten or twenty years ago), and not decrease it. Moreover if the forces are to reflect the society they serve, then they must (like society as a whole) have a larger proportion of graduate level people.
In today's climate, if you want graduate level people, you're probably not going to tempt enough of them in before they have the chance to exploit their opportunity to go to Uni. Any sensible young bloke who's worked hard enough to get to Uni is going to want a degree as a fall back position, and will relish the social, sporting, fun and UAS opportunities that a degree course will provide, rather than wanting to go straight from the grind ofA-levels to the grind of (waiting around for months on end to start) Cranwell.
Moreover, it seems to me that recruiting a bloke at 17.5 will give you someone who (generally) will often lack the maturity to hack many aspects of training (today's school leavers are much less mature, much less self sufficient, and on the whole have less nouse, grit, and initiative than their predecessors). It also seems to be being used to allow stupidly long holds, rather than in getting blokes to the frontline at an early age.
Of course a youngster will absorb training better and quicker than a 21 year old graduate, but if that graduate has done three years on a UAS, he may have other advantages that more than compensate.
As I say, the existing, mixed system has worked well, and I think that viewing a UAS graduate as being somehow inferior to a DE 18 year old (and discouraging your entrants from maximising their potential, maturity and qualifications on those grounds) is stupid, short sighted and counter-productive, and will lead to declining standards.