Cifer said:
As for the rest... Yes, there certainly are a few situations where you absolutely can't use more numbers. In those cases, +10% Astartes would be useful. But are there enough of those situations to justify the absolute nightmare that are Space Marine logistics?
One such situation would be Space hulks. Any kind of Imperial Guard unit would not have a chance of reaching the Space marines effectivness in cleaning one out.
And for another thing, I belive they are also a gigantic morale boost, basicly they are propaganda. When imperial guard fight the Necrons, the Tau, or the Eldar (or a big number of other races), they suffer huge losses, and things like that can really demoralise th Guard. Not just that regiment or pluton, not just that army, but the WHOLE Imperial Guard, if they know there are enemeis out there that the imperium cant deal with. But now, they can always fall back on "Well, we have the Space Marines, thus, we are going to win!"