Maybe back then, Dreadnoughts weren't the tombs they are today, but rather a more advanced sort of prostethic limbs that allowed a warrior to fight on where even prostethics and extensive surgery couldn't keep him alive? Maybe they were fully conscious all the time, instead of the death-sleep of modern Dreadnoughts?The Legions though, before the Heresy and the civil war it entailed? They didn't have such a massive shortage on men - why would they have to make use of such an option?
So being placed inside a Dreadnought was considered a high honor (you are so important that we want you to live on and give you one of our extremely rare, powerful machines) without the drawbacks?
But maybe the writers/artists just made a mitake.