The loyalists were in a no-win situation, but fought on regardless. This noble but ultimately futile display of loyalty was symbolised in the figure of Loken, betrayed and killed by one of his closest brothers. Bringing him back belittles that, especially as it was only to turn him into a cash-cow.
I understand what you're saying KF. But I personally feel that Garviel Loken did die there, in a sense. The man he has become now is not who he was, the trusting and somewhat naive Legionary Captain, the one who truly believed in Legion and Primarch and felt a little like what a real knight should be. Now Cereberus feels a lot darker, someone who has had his trust betrayed and won't be extending it again any time soon, someone who thinks that his and by extension his allies' efforts are futile and that all that remains is to make the darkness work for it's victory.
I say that Garviel Loken's original persona died at Istvaan and that symbolised, along with the deaths of all the other Loyalists (especially Solomon Demter whose death I found more poignant and heartbreaking than Loken's), the death of innocence and betrayal that sparks the Horus Heresy. Loken's return and who he has become is a foreshadowing of what will come, the same as the Imperial Palace becoming the Imperial Fortress, Dorn burning the Remembrancer records, Malcador forming the proto-Inquisition; these are all the beginning of the Imperium of Man we all recognize.