Think of it this way -
You raise a possible het male to try to prove him out - he never proves out - so you have an adult normal male, which you won't be using in your breeding program once you don't prove him out, and he has no way to "earn his keep" in your collection as a normal male.
You raise a possible het female to try to prove her out. She never proves out - but you still have a breedable weight female who can still be used in your breeding program to be bred with anything - recessives to make future hets, doms to make more doms, co-doms to make more co-doms - thus maintaining her value in your collection.
This is all from a business stand point when deciding what to hold back.