Layout for Book Publishing?

You are correct that having a FINAL (cropped and sized) image over 300dpi has no benefit. That is precisely why InDesign allows all images to be down scaled to exactly the resolution you choose.

The argument for making images larger than required is that it allows far more flexibility in scaling them up and/or cropping them in a publication later on. For this reason many designers use images that are very high resolution knowing that a section will become a detail later on.

Of course. I chose a Sony camera with “R” in its model name to take my photos. Overkill 98% of the time.