That’s true. I was playing Japan and UK attacked me UK2. I could send almost all aircrafts to kill off Brits without having to worry about US fighting me in US3.
I will offer up a rationale for the tac/tank combo. Think about Panzers and Stukas for a moment. The Panzers are used to pin down the enemy in known locations. Then Stukas are sent in to finish off enemy defensive positions before the Panzers blitz the line. If it weren’t for the Panzers pinning the enemy units, they would be harder for the Stukas to hit.