The TriStars were originally bought to fulfil a requirement to tank a C130 down to Stanley, for it to fail to get in, and tank it back. That requirement very quickly disappeared when MPA was built and the political climate improved but no-one wanted to admit it in case the purchase was binned. That is why the freight bay was filled up with nearly useless fuel tanks by Marshalls. Most of 216 at the time would have preferred the -500s to stay in their full civvy fit with 300 odd passenger seats and a usable freight bay.
This sort of incompetence was not confined to the military, though. The rumour in BA in the early 80's was that the TriStar was judged too expensive to operate by the accountants. They had looked at the fuel flow per engine compared to the B747, amongst other things, to work out the costs per seat mile. Allegedly, once the sale to the RAF had been agreed, someone pointed out that multiplying the fuel flow by 4 was always going to make the TriStar compare badly to the Boeing.