Generally, moderators are discouraged to decline flags without a strong reason.
- Clarifications given in September 2011 Newsletter of SE Community Moderator Blog look pretty straightforward:
Flags Too Often Marked
Marking a flag
[declined] was designed to deter serial abusers of the flagging system, but we find that this “slap on the wrist” is being used more often than is beneficial.
Flags should be closed as
[helpful] under most circumstances. If you feel strongly that a question was flagged in bad faith, it is okay to mark it
[declined]. But try to err on the side of clearing as
[helpful] whenever the user is trying to be genuinely helpful, even if you do not necessarily act on the flag...
Given that moderation is human activity
involving judgement call, deviations from above guidelines are inevitable.
As for the answers like one you flagged - ones that are barely more than a link to external resource (link-only-answers), these blatantly violate guidance given in How to Answer instructions:
Provide context for links
A link to a potential solution is always welcome, but please add context around the link so your fellow users will have some idea what it is and why it’s there. Always quote the most relevant part of an important link, in case the target site is unreachable or goes permanently offline.
By doing so, link-only answers set a bad example for Stack Exchange users, similar to how it happens with broken-windows questions: "why can't I answer link-to-X when link-to-Y answer exists".
Community benefits of removing (more precisely, converting to comments) link-only answers are eloquently presented here:
If a link-only answer is accepted, it is especially important to delete it (converting to a comment if the link isn't broken yet)... When a question has an accepted answer, it looks like it has a definitive answer, and there is not much point in looking for a better one. People who are looking to improve the site by providing better answers tend to consider questions with accepted answers as very low-priority. If a question has an accepted answer which consists solely of a link, this sends the wrong message, especially after the link breaks. Sure, the accepted answer might have helped the asker, but it's not going to help future visitors, and the community should not be penalized for that answerer or asker's failing.
Given above, it is not surprising that official FAQ -> Why are some questions or answers removed? specifically targets link-only answers:
Answers that do not fundamentally answer the question may be removed. This includes answers that are … barely more than a link to an external site.