Keeping young sign languages pure might not be in the interest of the communities that create them.
An interesting in-depth article by Michael Erard about village sign languages and ethics in linguistics. Excerpt:
Very young languages offer an opportunity to see how languages emerge and evolve – and therefore what the origin of all languages might have been like. But some linguists have wondered how pure these circumstances really are. They worry that studying one of these sign languages – which may have only a handful of users – introduces an outside influence that could alter its development.
So De Vos was sitting on her hands – deliberately not using signs from other languages – when she was in Bengkala. If there was any chance that she had changed the course of Kata Kolok, her research would be less valid, and its relevance to learning about the natural evolution of languages diminished. The only problem is that shielding a language like Kata Kolok for scientific benefit might not actually be in the best interests of the community that uses it.
Read the whole thing.
The article also comes with a longer reading list about sign languages, which is worth checking out.