In the case of worldtraveler303, 105k instead of 110k, coming with a huge risk in case of mis-connect, and since they already have an F ticket to HND, I suspect they are booking a PE connection now only because J is not available yet. If J becomes available they would switch to that and there is no mileage difference for ORD-HND-BKK F-J on separate tickets versus one ticket (if I am reading the chart right). For someone booking less than F separate tickets would be more expensive.

F-J on separate tickets would be a good option if the traveler wanted a stopover in Japan. If you don't have the 5k extra miles it would be your only option, but on a simple connection I would try to avoid separate tickets if possible due to the risk of mis-connect. That said, if I were in the same situation as worldtraveler303 (married segments currently prevent booking together and separate ticket available) I would book the separate ticket and look to change to one ticket if it became possible later.