Answers that are simply a link are not good answers. They should be voted down, commented on, or otherwise encouraged to be improved.
Answers that are a link plus a quote may be good answers depending on the question. If the question is answered adequately by that quote, that's perfectly fine. Note that in this case, it should be made clear that it's a quote (using the blockquote markup, prefixing each line with
> or using the
“ button above the editing area), and the source should be linked to, as otherwise you are plagiarizing someone else's work.
In other cases, the best answer might be a link plus some extra explanation describing how that link relates to the questioner's specific situation, or a link plus a summary of the contents of the link. And it may be the case that you need all three; a link, a quote, and an explanation or summary.
If the original answer doesn't do one of the above, we can use the facilities of StackOverflow to improve the situation; add a comment requesting that the person add clarification, edit the answer yourself, create another answer yourself with better quoting, linking, or explanation, or downvote the answer.
My answer to "Multiplayer timer for board games" is an example that does linking and quoting the way I believe it should be done. The question is pretty much already answered by an existing review, but I provide a link with credit, a short quote, and an explanation that expands on some of the specific topics asked about in the question.
Another good example of the link and quote style is my answer to "Differences Between Rijndael and AES" on StackOverflow; the question was answered adequately by a simple Google search, and several people just linked to the top Google hit, but my answer which provided both a link and a quote was much preferred over the simple links.
Here's an answer by Egg of P'an Ku, provided by linking to a BGG thread, that's a good example of the right way to do it. Provide the original link, a properly formatted quote, and a little extra explanation of some other thoughts or personal experience.