Yep, I’m not sure of any benefit that is conferred just by me putting 2 things into a PNR, though!
This is how I rationalise it and I may be wrong in my intepretation:
It boils down to who is responsible for IIROPS
Assuming that a Single PNR regardless of the airline combination, protects the onward sector is correct, the responsible party that protects the onward sector is usually the inbound airline causing the delay/disruption.
So a QF to any airline Single PNR, if QF caused the disruption, QF is responsible for the rebooking of the onward sector. I am not sure whether it delegates this to the onward sector airline or does it within its own systems.
In a QF to AA separate PNR, there is no mandatory obligation for QF or AA to provide onward sector protection, even if the onward sector is operated by a OW partner (Thanks BA

). However, AA, in the link provided by
@serfty, has a policy to treat the separate PNR as though it is a single PNR. I don't know if this is a bilateral agreement between AA and each OW member airline or a unilateral pragmatic policy by AA.
Others have mentioned that there might also be a "residual" partnership/relationship between QF and AA, that even without the explicit AA policy, travellers could request the assistance of AA to rebook them if the event of a separate PNR.
Ive found that if the itinerary was in the other direction, it is my experience that QF would (at the airport) assist a non protected disrupted passenger coming back from overseas to AU - without charging a change fee.