Natural selection improved the species by adding this redundancy in the first place. Mutations are very frequent and some of the phenotypes are critical/desirable for the offspring to survive. This way, the gene families serve as a protection against mutations on the critical genes, so that mutations on other genes can be passed on more frequently (without dying). By adjusting the relative sizes between families, the plant/natural selection can modulate the frequency in which mutations affect each phenotype.

Mutations to redundant genes are still beneficial to natural selection. The overall population will have a varied mixture of genes. While we consider that the phenotype is equivalent between all of them, some will have improved efficiency, resistance to pathogens, etc. These unknown phenotypes will only be known when the subject is exposed to an environment where that phenotype is beneficial.

Redundancy also helps the evolution process by making it easier to have mutations happen in small increments. One offspring could have a functional A gene mutated to non-functional A' gene and still survive, and then their offspring could have another mutation A'' which is better than the original.

disclaimer: not from a bio-science background, but all of this applies in the field of genetic algorithms