Lowering Moral can have a drastic effect on a player. It can cause them to play worse than they actually are. So it can be useful all the way from Run Defense (lowering moral of a ball carrier and making it easier to tackle them) to Pass defense if you are covering a TE u could lower there moral and just make them under perform ) I've seen LB with high AoI miss less tackles then they should have, and cover TEs that should have blown right by them.
Though it's almost impossible to actually SEE it work. Thats why people shy away from it....