They are extremely interesting deals. Just out of curiosity, are there any other airlines that ever offer anything even vaguely similar? Royal Brunei seems to be about the closest I can find.
Years ago (at various times) BI used to fly from BNE, DRW or SYD (at least) to Brunei, but since it stopped all these (not concurrently) and introduced the MEL route it seems to have often been the price leader.
One company you haven't mentioned is your own (QF), that occasionally has had 2-for-1 deals in J but a disadvantage is that at all times passengers must travel with each other if I correctly recall.
PR may have had low yields as MrMaxwell states (although fares are high at peajk holiday times) but unlike for instance CX, PR appears (like 5J) to be profitable overall at present. 5J allegedly is still unprofitable MNL - SYD and v.v. while PR (like many airlines) does not disclose route-by-route profitability or losses. It must be slowly carrying more connecting passenger traffic now that it flies from AKL via CNS, BNE via DRW and separately SYD and MEL to MNL, and is daily to LHR albeit at present with old A343s on that longish leg (this may change to a B773ER at some stage.) The connecting passengers assist a little with overall improved revenue generation.
BI doesn't always have huge numbers of seats available as its sole flight, while daily, is not on a high capacity aircraft (B788 - J 18 Y 236) and it connects to DXB, LHR, BKK, HKG, SIN as well as MNL to name a few plus the relatively small origin-destination passengers MEL - BWN and v.v. While all non-Australian based airlines suffer from some disadvantage in this regard, BI's four B788s are in full use daily (three BWN - DXB - LHR and v.v., one DXB - MEL - DXB) so when one goes off for maintenance a flight tends to get cancelled, and on occasion it can be the Melbourne one. Looking at the BITRE figures, BI is not necessarily full in each direction every day. Passenger awareness of BI outside of Melbourne (and perhaps ABX/ MQL and Tasmania) would be pretty low.
It's subjective but I have found the food better on PR in J than on BI, and the latter declines to serve alcohol (although staff will give you a mixer if you bring a miniature on board.)
D7's "J" offering, if one can call it that, is a cheapie version though some say quite reasonable lie flat bed but one must pay for various extras, and it's lacking the privacy found on many airlines in J.
The new J seats on PR may well be better than BI's, and better than MH's A333 J offering.
The mainland Chinese airlines do not advertise any deals they have well, and nor does CI from the attractive island of Taiwan, despite CI having quite a few TransTasman flights each week and then on to or from TPE.