Sure. You might, for example, embed an HTML card in a post that includes a reference to an image on the site. Ghost isn’t going to know about that image (since it wasn’t added as a normal image), but deleting it would mess up that use.
I’m not opposed to a button that permanently deletes images with a warning, or this being an optional setting that defaults to off. I’m opposed to the behavior changing in a way that automatically deletes images that don’t look like they’re being used. But I’m not the Ghost Dev team, so what I think is OK doesn’t matter very much. 