Many years ago, when trying to fit VC10 courses into the programme, I asked why the heck were we even contemplating course which had less than full trainee crews. The workload on the staff was almost the same when training a full crew as it was when training only 2 pilots, no engineers and no navigators because the engineer and navigator instructors would be required to operate, so that the student pilots wouldn't have to cope with possible non-SOP or substandard operation from 'guests'. But 'part crew' courses were such a waste of money.
"We've always done it that way"
Even the boss (not the most popular Sqn Cdr we ever had) asked 'them' why we didn't just arrange courses when there was a
need, rather than on some dubious calendar basis. He was encouraged not to ask again.....
You'd have thought in these days of IT that someone could do some accurate predictions for training course requirements.