Spec aircrew is a bit of a vexed issue, that's for sure.
From a 'greater good' point of view, I think it's great there's at least some mechanism to keep valuable experience in squadrons by breaking that 'must be posted after a couple of years, must be promoted' cycle, particularly when flying hours and opportunities for on some types are limited and it can be hard to really learn the ropes as a boggie prior to being posted out to a ground job (although I can't claim to be in touch with how things are these days - please correct me if that's not the case any more).
On the down side, does spec aircrew really achieve that aim, or is it just a good thing for those who get it (not that I begrudge anyone the opportunity, I sort of did it by default in the end with a combination of luck and some ducking and weaving!)