For BA, FFM redemptions are both cash flow and revenue positive. Due to the fuel surcharge, it provides both of these because we can be darn sure that the surcharge has a goodly margin of error built in. They step it up in bands to allow fuel price fluctuation within it. They also
It may also be the case that the person/family only have enough FFMs for one ticket and so they buy the other/s or they buy the base ticket and upgrade with FFMs. I have done both of the above at various times and so the cash into the carrier is certianly positive.
Also, there is no need to repudiate all the built up FFMs that are in circulation, as they know that the restrictions they place on using them (as we have oft discussed in here) means that many of them will never be claimed. This may be because of the limited seats available or because of the fuel surcharge. [Now just start reading the post at the beginning and you will have found one of the great beauties of BA's system.