IMO feeder are just entertain people to watch them eat. There are way to many healthy diet out there, why feed feeder?
Feeder will bring disease to your piranha, it may look healthy, it may bring out some color, it may look good, but it's not healthy for them in long term.
If you notice that some big rhomb have those disease call "hole in the head" it's from feeding to much feeder and live food.