Just to explain from my side of things (I'm sure I'm one of the worst offenders):
If you look at the way
Badges are laid out, they actually strongly encourage this behavior.
Marshall badge rewards 500 accepted flags
Low Quality Post and other reviewer badges are awarded at 1000 review items
So if I just leave a comment, no one gets any badge progress. If I flag it as link only and someone agrees, we both make badge progress. It seems to be the flow the system is designed to incentivize. Also, when accepted as
Link Only Answer in the
Low Quality Post queue, it auto-comments with similar material.
One counter-argument, I suppose, is that we may not feel the question should be deleted just because it may break later. Sometimes in the review queue, we see a link only answer where the accepted answer is, in fact, only a link. Often this answer is highly voted. It would be more consistent to respond to both
Answers with a comment, or both with a flag. As I understand it, deleting any post causes you to lose the reputation gained from it, and that seems harsh for something that was deemed useful.
If we are to post a comment, perhaps this comment can serve as a template:
Welcome to SFSE [user]! Can you post some of the relevant content from that link? Links tend to break over time making answers without details a bit useless in the long term :)
For a less green user, I might comment:
Can you post some relevant content from that link? Links tend to break over time, making answers without details less useful in the long term.