Stella has claimed her favorite perch, and Thomas has become complacent, allowing her to perch there as long as she doesn't make loud noises in his ear.
Luckily, Max does not also do this. She just bangs her beak into you until you acquiesce and pet her head:
1) they'd probably get better-quality preening since it would be done by a grey parrot and not a human
or
2) they'd probably get even more preening since they appear to have an insatiable appetite for it, whereas our appetites for this are quickly sated.
Quality and quality arguments, and I can't win this debate with them!
On an off-topic quick note, Stella's open wound, which I discussed here, has healed. I'm pretty convinced that she did not mutilate herself, but injured herself while flailing around her cage as she does not have feathers there to protect her skin. She has been letting more and more feathers grow in, so perhaps in time she can flail all she wants without opening up her chest.