Maybe indoors, in close confines, such as apartments with eight foot high ceilings, small rooms, aboard large boats--any place where the shooting situation is "confined". For example, aboard my in-laws' 42 foot boat, in the salon, bounce flash was ridiculous...the ceiling is so,so close to the flash that the ceiling is white-hot, and the fall-off is exceptionally rapid, leading to very ugly results...a "bare-bulb" type of flash, or a hemispherical-type diffuser would be (is) a much better approach than bouncing a flash. At longer distances, like say 15 feet or farther, or in larger rooms or outdoors, I don;t see any practical benefit to the Fong diffuser. It's like any tool; so many people say it's worthless...but that's an overstatement. The key is to understand the tool, and to know when to use it,and when to use a more appropriate tool.
A hammer is not a screwdriver. A chisel is not a knife. You do not hear people calling a hammer "worthless", no do you hear people proclaiming the screwdriver to be "worthless"...same goes for light modifiers of all types...there are situations where one,specific type of modifier will work well. I've just mentioned where I consider this type of lighting modifier to have actual, real-world applications: in close confines.