Hey I am a modder and created some stuff for the first Dawn of war game. I also worked on modding projects by coh1 and coh2 like the blitzkrieg mod and realism mod!
I am a big fan of the 40k universe and if relic made the game full unlocked for modding they can give the fans somthing back, let us say it will be a good marketing act.
If i can and relic give full mod support i will copy some models (units, tanks, buildings) from the dow game into the dow3 game and we have all races, its an easy process 2-4 monthes.
So do this relic! Otherwise you will lose all his old 40k fans (guys that are buying your games! dont foget that!)...