The importance of rare species depends upon your viewpoint:
- To conservationists, endangered species tend to be those of most interest - as are communities which contain those species.
- To theoretical ecologists, the most important thing is to obtain a robust mathematically well-behaved index. Since, by definition, there is least information on the abundance of the rarest species, they tend to reduce the reliability of species richness estimates. Given which, an index insensitive to rare species may be preferred.