Due frequent misunderstandings on when to do what and under which conditions, lets remember why we vote to delete stuff.
People have the privilege if deleting in order to be able to remove posts that need to be deleted. This is a quasi-moderation privilege so treat it with the same respect we do.
If we're talking about questions, the close system should be used first. This allows a poster to revise and reopen their question. And closing it without answers or positive score will see it automatically deleted. Duplicates feed into a SEO system which deleting can hurt.
If we're looking at answers, there's a bit more to do here in that it's far more common there's a "non-answer" posted in the answer section than a question asked that needs to be urgently deleted.
Save your votes for things that cause harm.
Leave the "does no good" for the system.
Since you are copying the meta SO thread here, we should also include the (very) important exception to the guidelines you cite. This is a screenshot, taken on March 16th, 2014 of the second highest voted answer from the same Q&A:
So, follow the rules above in general but avoid deleting questions closed as duplicates because they are useful "guideposts". The more duplicates there are, the wider the variety of possible queries that will lead a user to their answer.
You delete a post when the content no longer adds anything to the site. Posts are closed for a variety of reasons, so let's look at each close reason and whether they should likely be deleted:
It depends; Look at the context of how they are asked.
You'll want to keep the post if the wording provides another way for a search query to find the content. It's a bit like a "see also …" entry in an index.
Delete it if the duplicate does not add terminology or alternate phrasing to find the question. It may not be worth cluttering up the system with this exact duplicate. There are several poorly phrased questions that will not direct traffic to us.
Almost always delete it.
Off topic usually says "This shouldn't have been posted here in the first place." So, not about Ubuntu (read, Mint, kali, Debian, Fedora, etc.), solved bugs or bug reports, no reproducible stuff that nobody knows why or how was the issue solved, or EOL, just let it burn!
Delete it if it says "this is nothing but a 'broken window' and we should get rid of it." This includes list of question where the only information is about 2 lines of text per answer or wikipedia/internet dumps.
Always delete it.
Closing it before deletion provided ample time for the author (or community) to fix it up. If it wasn't re-opened by this time, it should probably go.
Primary opinion based:
See "Too broad".
This was adapted of Robert Cartaino's answer since some old terminology (not constructive, NARQ, Too localized) is no longer applicable. Also note, that an answer that is not an answer (if you strip the markup, doesn't answer the question) is not a party stopper to delete it.