Military vehicles can still be useful as disguises for infiltrating the military. They did this well in the first movie when Blackout managed to land at SOCCENT base disguised as one of their own choppers -- although they were confused because that chopper had been reported as destroyed, they still allowed him land under escort, which is all Blackout needed. Starscream also used his alt mode to access the US Air Force and even flew as part of a squadron before turning on the other F22s which caught them completely off guard. And while not a military vehicle, Barricade's form as a police car gave him access to where Air Force One had landed and also got him close to Sam.

So it's all about deception.

As for Sam's calcium intake, well in both TF1 and DotM we see him standing around drinking from a big jug of milk... so... there ya go. Drink yer milk, kids!