My dad has had to listen to me talk about linguistics for over 30 years now, but the other night he asked me what linguists say when people ask why it's important to maintain, revitalize, and reclaim languages. It was probably on his mind because of McWhorter's recent column on the topic. I told Mr. Verb about the conversation and he said I should post something about it. I certainly don't pretend that my answer is anything new - lots of people have said this - but it's worth repeating.
So first, there's the linguistcentric reason: if all these languages die, what the heck are we going to do with ourselves? But only linguists care about that one.
Second, people often say that languages express unique worldviews ... well, yes and no. I do agree with McWhorter that this can lead to the "when a language dies, a culture dies with it" mentality - which I've always found quite offensive towards groups whose languages are dormant. Having met a lot of incredible language activists at the 2013 DC Breath of Life whose languages are no longer spoken (or just beginning to be spoken again), I can tell you that they are totally still culturally connected. (See here for the 2015 BoL.)
But the third reason is social justice: colonization has caused loss after loss after loss. If I can put my energy into trying to prevent a further loss, then I should do it. As Crawford (1995) put it, "After all, language death does not happen in privileged communities. It happens to the dispossessed and disempowered, peoples who most need their cultural resources to survive." I've always thought that was such a good quote.
P.S. "Linguistic Justice is Social Justice" - see Colleen Fitzgerald's excellent post on this topic too.