I was shocked! A letter in a journal intended for professional linguists suggested that disappearing languages be allowed to die. I hope it was ironic!
Perhaps I am biased and maybe there are people working as translators and interpreters who are not as fanatical about languages as I am. Yes language is about communication and our job is to facilitate communication between speakers of different tongues. Certainly communication would be easier if we all used a common lingua franca, which in the current day and age would be English. However, the very fact of a language being endangered indicates that its speakers habitually use at least one other language.
Some languages may decline because another language is seen as a higher status. Perhaps its use opens up educational and employment possibilities. Nevertheless this should not diminish the value of the other language, which may have taken thousands of years to develop and represents the cultural development of its speakers. If the language falls out of use any written records and literature that might exist will end up in the hands of a few priviledged individuals with the ability to read them. In some cases this skill might disappear too. In the June issue of Current Archaeology there is an example of just such a case. Linguists from Exeter University have found evidence that symbols on Pictish stones are a simple written language.
Of course the disappearing language may not be a written one. In which case it is even more important for it to be documented. Even if such a language did disappear entirely vestiges may remain in place names, names of people, traditional foods, implements etc.
People identify with their language; it is an important cultural reference point. In fact language is constantly changing, as each generation adapts the language to suit its own purpose. As we get older we note that younger people seem to speak a different language, inventing new words or slang and changing the meanings of words, e.g wicked. In work we develop our own jargon and the ability to speak it marks us out as members of the group and sets apart from those outside the group. So it is should come as no surprise that people want to keep their own language alive. Personally I applaud all attempts to do so.
Cornish is a good example. The last person to speak only Cornish died over 300 years ago. Later speakers were bilingual and the language never actually died out. Just take a look at this website.