I sympathize with your viewpoint, but I'm not convinced it's correct. If there were a downvote, I
might have downvoted it, but not flagged it. However, there's only a flag. I try to use the flag to mean "off-topic" or "very low quality", and preferably both. I think the post under discussion was both -- it certainly wasn't technical or informational, and was it really newsworthy or intellectually interesting to anyone? (Obviously there are exceptions to every rule, but by "anyone" I hope I mean "the vast majority of the readership.")
I really don't like the "point out the flaws" approach. There's no way I would pollute a comment thread on a reasonable post with a discussion of why I think the post is no good, unless it was really bad and I was just trolling. I think that would lead to a pretty undesirable level of noise if it were commonplace.
I guess that my feeling is -- in a community with no full-time moderators who can look at every post, users have to shape the community in a way they consider reasonable, and in my experience online, taking out the trash is a really important part of that. There might be some unfortunate subjectivity in the process, but I think it beats the alternative of doing nothing.
(Anticipation: You may say, don't flag anything, but upvote many more things to indicate what you prefer. Unfortunately, there aren't really a huge amount of great posts that I feel stand out from the average decent posts, and I want to be able to really encourage the great ones in particular. Upvoting so many posts would seem like I was diluting my real preference, so I don't do it; I reserve the upvote for the top 10% or so of my quality range. I don't know if this is really rational, though.)