I feed back everything that I don't use, in one way or another...if my laying hens are free ranging when I am processing, they very quickly catch on to what is going on and are hanging around waiting for the 'goodies'. By feeding these parts back to the flock, I'm using the parts that I wouldn't otherwise, and in a way that adds back to the nutrition of the flock. Some people may be concerned about diseases, but I figure these birds have been living together their entire lives...if one has some disease, odds are the others already have it as well. Now, if I was processing birds that hadn't grown up on my property, that would be different and more risk, at least in my mind, of introducing diseases.
Letting the layer hens clean up also saves me work...as small bits and parts wash off the table between birds, they take care of it, so I don't have to worry about any stink building up from random bits and pieces being left behind after processing day. Kind of like the toddler that loves to 'clean up' under the kitchen table after dinner LOL. As I have parts that I won't use during processing, I toss them into a shallow dish that the layer ladies pick through -- once I'm done processing for the day and they're done picking through, the rest goes to the dogs, and anything they don't eat goes out into the woods for the wild animals.