Thanks for the replies so far, some food for thought. I think I understand the need for aircrew who are SNCOs, at least as far as the RAF is concerned - authority, recruitment and retention being among the reasons.
I also accept that these reasons are equally valid for the airborne FC / AT / IA SNCOs, but what I really want to know is where is it written down? Trap one thinks it might be in a DCI from around 1990, any more details? Somebody somewhere decided that these trades would be represented by SNCOs when flying, surely that decision was written down to provide the authority for selection onto the E-3 or Sentinel?