As has already been said, PAS is not the same as the old Spec Aircrew scheme where you got long term stability in a flying job for extra money - win/win/win!
PAS is designed to keep aircrew with the right qualifications in to do flying related posts.
Whilst it will vary between FJ/RW/AT, the truth is that at SO2 level there are plenty of desk jobs, the vast majority of SO2 flying jobs are for those moving up through the system and the chances of you occupying a Flt Lt flying post as a Sqn Ldr are remote while there are empty staff posts.
Certainly in the FJ world if you get a flying job you are very lucky (or have just the right quals at just the right time). Haven't heard that it is as bad as "only ground tours" though.