All birds of good quality, that are close to the "ideal budgerigar" are valued. However good quality "normals" are always in demand for breeding. Normals include Normal Greens, Blues and Grey birds without other mutations such as a pied gene.
A normal blue or green is required more for breeding as it will assist in producing better quality chicks. That is why top breeders reccomend pairing a spangle (or other mutation) to a good normal.
Many breeders 'specialise' in a particular variety, such as inos or spangle etc, and these varieties occassionally need to be bred with a good normal in order to increase their size and other qualities required in a show bird.
Budgerigar Shows, of course, have the full arrange of mutations.