P: XMP sidecar files to be availeble for DNG

This is why I stopped converting my RAW files to DNG’s during import.  the backup overhead was not worth the 10-20% reduction in disk space per image

These are among the reasons why I have for years avoided converting my camera Raw files to DNG at import (or subsequently). Instead, those remain in the proprietary format for the camera used. LrC does not store edit metadata within that, therefore a separate XMP is employed.

There is a preferences setting as to whether (very exceptionally) corrections to capture date applied in the Catalog should then be written out into a proprietary Raw - not only into its XMP sidecar - or whether LrC must even in those circumstances leave the Raw completely alone..  

Many people prefer the reverse: they actively like seeing no separate XMP file alongside a DNG converted image. That is among the reasons why they will advocate converting.

As a complication to the above simple system - some cameras shoot natively in DNG. Also some recent LrC tools generate a separate quasi-Raw file which includes some processing, and this is always saved to disk as a DNG (without giving any option).

So to manage those cases: I would agree, the option to force external XMP only seems like a sensible option to provide. This will, though, require a review of XMP file naming since currently, if you see a Raw and a JPG and an XMP all reflecting the same filename otherwise, the XMP is safely presumed to associate to the Raw. If JPGs might similarly receive an XMP when metadata was written out to them, such a presumption could not be made!