Why do we value species? What is so bad about invasive species? Why value biodiversity?
Here is an attempt at a partial answer.
Invasive species have been introduced to a new habitat by humans (or by natural means).
They usually do not have a predator in a new area nor are the pathogens in the area very adept at infecting them. So, they may proliferate rapidly and compete with the native species. They may also be ferocious predators as their preys do not have defenses against them.
system= more than one species
Advanced creatures such as us cannot live without other creatures, like plants and bacteria to support us. (yet)
If you want to see what invasive species can do watch this video, http://www.youtube.com/watch?v=Bk-9ndL3ykM it has some descriptions on what the invasive species homo sapiens and their domesticated cows do to their planet earth. Wasting resources basically. Displacing other animals and plants, monopolizing the earth. Monopoly or monoculture is not good. For if the monoculture fails all is lost. If you have diversity if one harvest goes bad it's not over yet. It's basically all about stability and sustainability. Without either advanced lifeforms cannot exist.
I don't know much about the field of conservation but I'm giving a try...
In the following, when I say conservation I mean: "Human decisions and actions in order to protect the ecosystems". It has not to be confused with the science of conservation biology.
I think that the question you're raising shows the paradox of conservation. The maintenance of Biodiversity occurs through a dynamic process of speciations and extinctions allowed thanks to mutations, selection, population structure and movements. Standard ideas of conservation is not to preserve these processes but to prevent extinctions.
Quite a lot of many in conservation is aimed to save some big and representative species. Species for which we, as humans, feel some tenderness, love and pity for their fate. I am not saying it is bad to protect this kind of species I just want to emphasize that if we conserve it is not only for economic issues (or for the survival of our species) but also because we have ethics, because we feel compassion and guiltiness with nature of with some species such as the panda, the deers, the lynx, the wolves or because we feel compassion.
I also think that our categorization of living things into species (which is quite an subjective categorization) might bring us to take some decisions that do not fit economic issues.
For those who speak french well enough, here is a serie of podcasts (follow: "La biodiversité - plus qu'une simple question de conservation/") that might interest you. Especially the one given by Pierre-Henry Gouyon, which pretty much explain (much better than I did) most of the ideas of the last two paragraphs.
The question of invasive species is particularly interesting I think because there are especially a lot of invasive species today, first because ecosystems of changing (mostly because of human's activities) but also because human means of transport allow species to travel very far and become invasive. I assume (although I have no reference!) that the current number of invasive species is particularly high and just like any other disturbance in ecosystem, we can hardly predict what will the consequences. So shouldn't we make sure to keep this number of invasive species to a level which is closer to what would be found in nature in absence of human?