What should be our policy towards link-only answers? In other communities I know, for example SO, they are not welcomed, because the link may be dead in a few years, so people are expected to provide the solution, and post link only as source reference or the extended lecture.

Here, however, questions are mostly about available resources. If the resource is no longer available, there's actually nothing to do with the question if the resource is not available elsewhere. Often the link is something that the OP is looking for.

For example, this answer to my question: Downloadable archive of weather conditions for Europe? posts a link, which is something I have requested. I think that such answers are valuable. But the question is, should we expect, that the authors would give some more informations about the resource, a short description or summary, instead of only giving a link?