It would be nice to be able to have more control over how to display the fields in (custom) posts and pages.
The three I am thinking about are:
The ability to group field groups into one meta box with location rules (very handy when having several custom post types with shared fields but not all types may use them all). – Another option would be to add the location rule into the field.
The possibility to add fields into existing meta boxes (eg. the featured image box)
To be able to hide the title of the meta box and/or field (seems redundant when it’s a field group of one field)
The main reason is to avoid cluttering the (custom) post or page edit/creation page without having to recreate all fields just to be able to have them into one meta box.