It's curious how people don't seem to think the primarchs coming home would do any good. I think in the unlikely event of one of them returning humanity will be greatly strengthened.
Many of the primarchs left to undertake titanic quests in the name of mankind the fruits of which will be huge deal breakers albeit far fetched. Going on the loyalists charachter I don't believe they simply lost hope and wandered off. I think it's fairly logical to assume they went out looking for something and won't return until they find it, the alternative being death.
With this in mind if Russ comes back alive humanity will presumably gain the "tree of life", should the Khan re-emerge humanity will learn how to defeat hordes of Dark Eldar on their own turf and gain what would probably be a mastery of the web way and if Corax come back they would gain knowledge of how to survive and combat the warp.
There's also the possibility of a loyalist primarch returning someday with the remainder of humanity united under his banner (lets not forget that although the crusade was nearing conclusion there was still plenty of exploring and re-uniting to be done) or maybe a fully functioning STC allowing the imperium to construct pretty much anything it wants from anything.
There seems to be a commonly held belief on this website that returning primarch would be rejected to which I would say if a primarch returned at the imperiums hour of need and when all seems lost with any one of the weapons that they set out to achieve no amount of imperial dogma and corruption is going to stop what's left of humanity from embracing them as saviours.