Isn't that enough? Highly biodiverse environments like tropical rainforests are treasure troves of thousands of fascinating species that we don't know much about yet. No doubt we still have a lot to learn from these environments.
Biodiversity sustains ecosystems, which, should they collapse, could very well take us with them. Due to the incredible complexity and interdependence of these ecosystems we simply don't know what the results of collapse will be. Maintenance of biodiversity is therefore a risk avoidance strategy.
Additionally, I'll mention that I agree with blaabjerg's response: it's worth maintaining biodiversity for the fun of it.
Yeah, I could use "incredible complexity" on something else like, let's say for example, climate and meteorology. But instead of being conservative about what we pump into the atmosphere, we could just dump in it like that's what it's there for. And see where we end up. I don't think it's at all unreasonable to draw a parallel with that and ecosystem complexity.
Corollary: I don't see how a broadly conservative approach to managing biodiversity can be discounted because it isn't specific enough.
