It's a cultural artifact so your question, I think, is equivalent to asking the value of preserving any cultural artifact, e.g. a Greek kylix or an Inca quipu.

In addition to the above pure artifact preservation aspect language (when paired with a lot of text, this is not always the case, e.g. Etruscan language) can make available vast data about a community. When Asyyrian was decoded and the great libraries of tablets were discovered (e.g. http://en.wikipedia.org/wiki/Library_of_Ashurbanipal), our knowledge of the culture of the Ancient Middle East increased exponentially.