Only JFK, ORD and EWR in the aforementioned examples ae back to back trips. The rest attract a variety of box or destination payments. JNB is a prime example of where Mixed Fleet could be utilised as it could legally be operated as a single night slip, which is exactly what Virgin do with similar timings. I have heard similar justification as Hotel Mode as to why the two fleets are segregated. I would also speculate that offering two different service levels on a specific route might cause commercial issues. Mixed Fleet are consistently scoring double digit increases in customer satisfaction in the surveys on the long haul routes they take over.