I have heard a comment about this from a very well-placed source - so it may be a rumour, but a very well-placed one. I recall the discussions several years back ove whether it was legal to provide taxed but not pensionable pay (eg flying pay, submarine pay, sf pay etc) and a legal challenge was mounted and failed due to incorrect process - meaning there hasn't actully been a ruling on it yet.....
Nevertheless the motivation for being on PA is more than just financial. Essentially you could serve to 55 as a flt lt and retire on a Gp Capt's pension after a career of flying without getting involved in 'greasy pole' politics. Not an option for career streamers, even if the pension is enhanced.