I like to think that the Emperor took the long view, and saw that if Dorn had his way, there would be this uber-force sitting on Terra, just waiting until Horus knocked on the eternity gate.
Corax basically wanted to hit back not just for revenge, but because it never suited him to sit still, wait in a bunker, and THEN start killing. The Raven Guard were perfectly made for hit and run, cloak and dagger. If Corax spent the rest of the Heresy running around laying the smackdown on small, isolated hosts of the traitor legions successfully, then he would've made quite a dent in their numbers over the better part of a decade. It speaks VOLUMES that all the canon and fluff we can muster basically states that Horus knew about the approach of the Wolves and the Angels, and knew that he couldn't win a standup fight if it kept going.
So, I think the Emperor relied on the fact that his sons fought differently, and willingly gave Corax the Genetech, so that he could fight as he was made to, and so that when the traitor legions reached Terra, they'd already be weaker for it, if only incrementally.
-Proteus