Thanks to cko (who lent/loaned/leaned me her copy of Crystal's Language Death) I was finally able to read Chapter 2 Why Should We Care and Chapter 3 Why Do Languages Die. I have to say, I found the general writing style disappointing. It’s a lightweight volume that reads like it was pasted together from notes and speeches (which it may very well have been). He tends to make the same points over and over, in no systematic order.
Only 3 main points relate to why language death is bad:
I. Languages are like an ecosystem = ecosystems have mutually reinforcing relationships between members/elements (i.e., hurt one, hurt the system)
II. Languages are repositories of data (i.e., we can learn stuff from them: history, culture, linguistic feature space)
III. Language = identity
It is argument (II) which I find most compelling, and the one I agree with most readily and without debate. Yes, I agree that all languages have unique linguistic properties that are well worth studying in themselves. But just because we find valuable data in every language does NOT mean we should stop language death per se. we need a broader understanding of the system of language interaction and language evolution, otherwise zealously stopping language death may be as irresponsible as zealously causing language death. Like a protected species over-grazing or over-hunting a locale, language over-population may serve some ecosystem harm. We just don't know.