Now after they raised the supply and fuel use off mobile units that is indeed a good point. But before that the manpower problem had the same solution, more Armor and Artillery. Artillery is still a very competetive solution, and building 500 artillery brigades is far from cheap for any nationIn my opinion, the importance of high Practicals is often over-rated in single-player games. The effective size of your military isn't usually limited by how fast you can build them. More often, Manpower, Fuel or Supply efficiency is a constraint.
But due to how the airforce works currently More IC for planes on your side means less Manpower left on the enemy side. Given how easy it is to win in singleplayer I wouldn't say thats a good argument actually, you can do ALOT of things wrong and still win against the AI... that does not mean one should strive to play less efficiently.