Soldiers are cheaper then tanks and can carry more firepower per space on skyranger, rookies are pretty cheap when compared to tanks, the only reason to really use tanks are using them as mobile shields for important soldiers, although really you can also use rookies as mobile meat-shields for your vets as well.
Rookies are indeed nature's kevlar.
Well, no, while HWPs are far more expensive than rookies, when used properly there's a place for their exceptional mobility and staying power in the form of armoured recon. Firepower isn't much of an issue since, ideally, you'd avoid exposing the platform, and use its weapons only when there's no soldier that can take the shot
and it's imperative someone (or something) takes it that turn.
HWPs really shouldn't be judged like line combatants, so to speak, because their high cost makes them non-expendable.