Dick and others, A single rare allele is unlikely to be of any use to the future evolution of the population.
Most alleles are probably slightly deleterious, nearly neutral genes.
However, much of the quantitative genetic variability that phenotypic evolution depends upon may be due to such alleles. So it would be dangerous to a population to lose many such alleles. Most importantly, a single rare allele is likely to be a clue about other alleles that we are not assaying for. A subspecies or local population may well have a wealth of odd alleles that we can only guess at. Such genetic variability may be the salvation of the species in a rapidly changing world.