Like some others who already posted, I usually do it for the confirmed upgrades. I go to mainland China a lot, but there are almost never confirmed upgrades available when I book the ticket (usually anywhere from 4-10 weeks in advance). But, one can often find seats for confirmed upgrades into ICN or even HKG, so I will add an extra leg to one of those places so I can buy a B fare and upgrade with miles. It is worth it, even though it adds time and (sometimes) necessitates breaking up the trip with a night at an airport hotel. Then, I just buy a C class ticket from there on Air China or someone else.