Twin Trainer
I have no doubt that different departments of the military will provide explanations to justify their own ideas of how to manage their budgets and sometimes there may well exist some very spurious calculations or at least some illogical math models.
So essentially one department or entity can justify any fleet.
What we found in Canada was that in the end we finally did introduce a twin for pilot training but retained the single for the beginning of pilot introduction to helicopters.
The thought process being that without any singles in the fleet no one could see the point or justify the need to maintain strictly a single engine trainer. But no one was willing to let go of the essential skills and foundation that it provides. So a mixed fleet was retained of Bell 206 and 412.
Pilot do their initial training up to their first basic handling test which includes autorations to touchdown. They then move on to the twin where we introcude crew concept, IFR, NVG and earn their wings in an environment they will see at their units.
The advantage has been that the squadrons and their respective type schools receive a much better candidate and the costs savings have shown up there. So there can be a savings passed on which can justify the loss of the single in the basic phase.
It's becoming more and more common that militaries do not have singles anymore so if that is the case, why spend so much time and money training on one. In the case of training only on singles the units inherit candidates that are simply not ready. Type schools or squadrons spend too much time on larger aircraft training them to fly in a crew, hone their IFR and learn NVG. Having a twin or mixed fleet with a twin during the basic portion to graduate pilots with those skills can save a lot of money at the unit end of the production line. We have found this is a better model for a modern military.