Razib Khan: "Linguistic diversity = poverty."
I'm sympathetic to recognizing the real loss that accompanies the disappearance of a language from the world of speakers. The "unique oral history" and "lost in translation" ideas are true as far as they go -- the value of folk art and oral history is that they enable social relationships.
But most communities of a few hundred speakers don't have a Beowulf. Unique perspectives and unique history, to be sure -- just as every Rembrandt is unique. But every Rembrandt is not the Night Watch. Most unique perspectives are about the speaker's life. At some point we can't learn the stories of all our ancestors anyway, because there are simply too many of them. Obviously I think we should enable people to learn about their history, yet we can't keep communities pinned like butterflies in a cabinet of curiosities.
Human language communities in prehistory had a few hundred to a few thousand speakers. Those communities shared the same basic social lives and needs. Ninety-five percent or more of all those languages were lost -- and those remaining have mostly come from a handful of languages less than 10,000 years ago.
I read in the Rijksmuseum that art historians figure more than 95% of the work of artists from the Dutch golden age had been lost or destroyed over the last 300 years.