I think that the default backup behavior is useful: it fits the idea that the backup is more important than the new file, and it gives copy-on-write powers to hard links. If Emacs sees the file is version controlled it will not back it up by default, which also makes sense. The option to change defaults is there and may make sense for some users, like the author of this page. Having hard links to files that you edit and don’t version control and don’t want a copy on write behavior is potentially not a good idea, but if/when you need it, you hopefully thought through your editor quirks as well.
This one struck me as something that elicits a bad knee-jerk reaction despite there being a good reason for it. I seem to vaguely recall something regarding moving vs copying related to file system atomicity, but can't remember the details right now.
Yes. The move rename operation is atomic in any reasonable file system. With the default behavior Emacs can always keep the previously known good data in place even under power failure, disk failures, etc. if the new file is corrupt you go back to the exact old data (no bits moved in the disk.)
