I don't agree that it is perfectly fine to use the moderator flags in matters of voting-to-close.
The Moderators would be the first to say that they are perfectly happy take a look at questions that need attention. That is admirable, but what you are requesting amounts to a supervote-by-proxy, and I don't feel that is appropriate.
Flagging should be reserved for egregious problems (blatant spam, illegal posts, profanity, etc). If you feel a questions should be close as off topic, that's what voting is for. If a questions doesn't get the votes, it shouldn't be closed. Otherwise, all you are saying is "my one vote should be enough" and that's not right. If the Moderators have to act on these I-think-the-community-should-close-this flags, then the community is not doing their job.
But don't extrapolate my convictions out to absurd proportions. Certainly there are scenarios where manual intervention may be warranted: Problems with a very old question fallen out of public view; illegal or personal issues where the community chose not to act appropriately; etc. In those cases, certainly, flag for moderator attention. Moderators are the "human exception" handler. But moderators should not be called to intervene in routine system community self-moderation unless it is absolutely necessary.
...I need to document what it is, exactly, we expect moderators to do. The short answer is, as little as possible!