Let me see if I've got everyone's points down right.
1) Sentimental / Aesthetic Concerns
TK: The languages (or species, in the biological example) themselves are reason enough.
SHG: Ask an Aboriginal elder who can't teach young people the traditional chants and stories, in an entirely oral culture; or who can't pass on the true names of the sacred sites. ... Ask a Chechen, or Uighur in Western China, or an Amazonian native who has seen his entire way of life completely dissapear in a mere decade or so.
z: Wouldn't it be boring, ...
hrh: That would be a loss of incredible proportions.
SHG: ... for fans of Universal Grammar, we need as many sample languages as possible, otherwise whatever grammar we have might not truly be Universal.
3) Cultural diversity for pragmatic reasons
TK: Why is it important to try to preserve different species of animals, even if they're genetically almost identical? We've got humans, what do we need chimps for?
(Because more diversity in a group/system means a greater chance to survive.)
Iscun: The preservation of those regional languages is part of a bigger plan: the upholding of the culture and heritage of the people who speak them.
SHG: Language is culture. Every language that dies takes a culture with it.
z: if local difference isn't important, why shouldn't the whole planet look like suburban America, so that Beijing and Barcelona and Bulawayo all have nothing but the same malls, the same restaurants, the same language, the same music, the same TV programs? Wouldn't it be boring, and possibly even politically and culturally dangerous? Variation creates flexibility and new ideas.
Anyone feels that I have left you out / misquoted you?
(Mercator: don't worry, I'll go on to your point later.)
Winter is coming