What are some positive and negative consequences of a loss of biodiversity?

Pro: new niches become available; species will have the ability to diversify and occupying those niches.

Con: you have to kill off some species for those niches to become available.
Con: in terms of biOSphere stability, it's always better to have more diversity than less.
Con: in terms of gene survival, it's always better to have more diversity than less.

I'm not sure there are any pros worth the cons of this...