Marc, If you don’t make changes to XMP properties (ratings, keywords) in PhotoLab, there’s no reason for PhotoLab to write to XMP. If you are making changes to XMP properties, I’m afraid you and everyone else like me who do not use PhotoLab as a DAM do want those changes written. Otherwise we don’t have any kind of sync with our existing applications.
Your point that DxO should proceed with considerable care here not to damage existing data (zeroing XMP files for instance) is well taken.