Two main views on the importance of macromutations:

• Developmental macromutations are evolutionarily inconsequential. They may arise from time to time, but will always be selectively disadvantageous because they are introducing a gross change into a fairly well adjusted machine: they will therefore soon disappear from the population.

• Macromutations in the genes controlling development are the source of many evolutionary innovations. The mutations are often known as hopeful monsters. The idea is that radically new phenotypes are produced by mutations in developmental genes, and these mutations are the mechanism by which new major groups evolve. The idea has some modern supporters and, as a rare theoretical possibility, it cannot be ruled out. Maybe selectively advantageous macromutations do sometimes arise. However, there is no evidence that they do, and theory suggests they would not. The theory of evolution by hopeful monsters should be no more than born in mind as a hypothesis; it is implausible in theory and unsupported in fact.

Richard Dawkins gives his view on the subject.