Why do we so frequently use mutant alleles to determine structure and/or function of the wild type? ...

Mutant alleles create alternate versions of an existing protein that can provide important information about the normal function of the protein. When the wild-type function is lost, you can determine the phenotype of the organism and gain insights into the role of the protein. Similarly, if two different mutants phenocopy one another, you can identify redundant genes or members of the same gene pathway. You may find that mutations in one region of the protein may show no phenotypic effect, while other regions are highly important to normal protein function. Thus, you can begin to map the important conserved functional domains within your protein of interest if you have mutant alleles with different mutations in the same proteins.