Is it appropriate to answer a question by simply linking an existing article and possibly giving a short summary as to why that answers the question?
I agree with myzplk and Bryant, but I think that a sort of "annotated bibliography" approach should be encouraged rather than just a bare link. Someone providing a bit of context or a snippet of review (e.g., "This link is to an article by so and so that sums up the issue nicely, especially the third paragraph" or "This has dramatically helped my games go more smoothly") is much more valuable than just the link, and helps make the site something more than a human-powered search engine.
I think so. The idea is to get questions answered and build knowledge, right?
Absolutely. Duplication is not building knowledge when a link will do.