Every species is a part of an ecosystem and contributes in some way to that system. All parts make up the environment, like a puzzle if you start removing pieces at some point it will become unrecognizable, and an environment that has lost too many species will break down. I think there are few who would suggest that the environment does not contribute to the quality of human life. To suggest that any species is unnecessary is to demonstrate a lack of understanding of the environment.
Doc, You seem to suggest that we should preserve all species because everything in nature is interlinked in myriad ways and it is difficult to predict what kind of drastic impact would result from the disappearance of species. Generally speaking, that might be true. But what if we knew?
One potential scenario might be that there are only a few of a species still surviving? So few that it really would not make a difference to the anything in the environment? What would motivate preservationists in that case to spend time and money to ensure the remaining few of them survive?
