There are multiple reasons why answers with just a link are bad:
- Firstly the links can go bad. The website might change its structure (ESRI resources, I'm looking at you), or even just shut down.
- Even if the page exists, and the software/tool is available from that page, the answer should explain how to solve the issue. If I open up a new software, it might have a steep learning curve, and just the link will not be useful.
But does that mean that every answer should be lengthy? No. Writing a lengthy tutorial with screenshots and diagrams is an labor intensive process, and you can't really expect every answer to be like that. (I don't know how Whuber manages to consistently write such detailed answers; Every time I see one of his answers, I take pause and read it with attention, and learn a great deal)
We should strive towards a middle ground.
Answers should not be just a link. At-least a paragraph or so should be written about why and how that particular link will help the OP. Someone shouldn't be penalized just because they didn't write a long tutorial with images and diagrams.
If someone wishes to write a lengthy answer, then that is good behavior and it will automatically be rewarded with upvotes from the community.
And If you as a reader feel that an answer could be improved, go ahead and leave a comment asking the Author of the Answer to expand it; You could also post a new answer based on the existing answer, adding as much details as you like(Providing due attribution).