That would be a nice idea if demonhost fight till end and not quit after 2 stolen souls. Demonhost should be and is a stronger creature than any of these “trash” classes we play. According to this, it sounds far away safer to avoid demons.
See the demonhost as a balrog but you’re not a Maiar nor a superior Eldar. So, that would be a harsh fight to “kill” them.
If they become a way to gather some resources, may they fight till the end which means whole squad can be killed^^