I wouldn't say that the exact calculus for infantry on attack vs. defense is accurate, since there's still pinning hits, retreating hits, aiming at favorite targets, and numbers of attacks to take into consideration. Even though infantry on defense with entrenchment have an advantage, they still seem to be limited to 1 attack per man. This means that human wave attacks do not need 4-to-1 or 8-to-1 ratios to dislodge the defenders (although human wave attacks are still inefficient, I've tried them many times in the game engine). Sometimes well-staffed high-morale human waves only need 3-to-1, or even 2-to-1 ratios if the enemy doesn't have machine guns.
I also disagree that infantry's only purpose for riding on armour is to serve as armor for the armour. Tanks typically have poor recon values, which means that they can easily suffer from surprise. And, although they do have many attacks each, they still have to make tries at favorite targets. Infantry have different preferences for favorite targets, and thus may be able to get rid of extraneous units so that tanks can focus on attacking more important targets.