[Reply to a message mistakenly posted in the SDK forum:
http://forums.adobe.com/thread/1158444?tstart=0]
Adobe has stated that LR 3.4 and later follow the standard "Guidelines For Handling Image Metadata" from the Metadata Working Group, which incorporates the EXIF and IPTC standards. Adobe continues to make incremental improvements to LR's metadata handling, so I'll just focus on the behavior of the current release, 4.3. Following the detailed steps you posted:
LR 4.3 does not add Photoshop:Urgency to the .xmp sidecar. As you've observed, Photoshop:Urgency is a deprecated field not part of the IPTC Core, so that makes sense.
LR 4.3 does copy the XMP field MicrosoftPhoto:RatingPercent from the .nef to the .xmp, but it doesn't update that field when you change the Rating in LR. Both of these behaviors make sense. RatingPercent is not part of the standards supported by LR, so of course it won't update that field. But it is part of the XMP metadata in the .nef, and tools should in general preserve all of the XMP metadata, even if they don't use it or know how to interpret it (XMP = "Extensible Metadata Platform").
