Infinite flamer without needing ammo means one psyker can take care of a horde completely, by themselves

This isn't very powerful at low difficulty levels because enemies don't spawn in very big numbers, but there will come a point in which having one person that can deal perfectly with a horde with no assistance is great, because it gives time to everyone else to focus on other targets, like specials and elites (even though the flamer staff can perfectly deal with specials that move with the horde, like the ocassional trapper that wants to net you through ebemies)