Any forces held back for base defense aren't engaging in the main skirmish though, and that's a significant disadvantage.
Whereas the Orks are very unlikely to care if their base gets smashed up, cause they'll just have the gretchins hammer it back together when they get back.
Though again, Looted Valkryies might just give the Orks the air advantage. Valkyries took down even waves of scourge due to their tendencies to bunch up, same with devourers and mutalisks.
They were the penultimate anti air unit, and looted ones always bring more fire power to the game.