"Why pay them on PA terms when you can have them for less..."
Well, more pay was the theoretical reason for the PAS at that time; give them more to keep them in. Sqn Ldr pilots on the CS, who were not good potential for either command or SO1 staff work were not getting the advancement, but stuck forever on an SO2 desk to age 55, were leaving the RAF, quite understandably. By giving flying related status to a staff appointment means that the Sqn Ldr would move to the PAS, fly a (peripheral) desk, and get a better than average Sqn Ldr pension. Some would even go back to the FL to gap any areas. I believe that was the theory, but I don't know if the offer or application to transfer to PAS still exists at age 44.
If its possible to feel sorry for a 55 yr old Wg Cdr on the CS, without Fly Pay in his last 3 years, his pension will not reflect the extra work he has put in, compared with the relaxed Flt Lt/Sqn Ldr on PAS Level 35. I don't know the figures, but there is some inequity there. I foresee a lot of senior officer pilots, in their mid 40s, who can neither get promoted to Gp Capt nor get on the PAS, pack up and leave the RAF.