Don’t fence me in
OK, I am being mischievous by even suggesting Apple add to Aperture an “export to Lightroom” feature. And I wonder if anyone would ever find it worthwhile to map one metadata driven editor's XMP to another's sidecar format, or even one day use scripting to pass at least some data between Aperture and Lightroom.
My point is about the vulnerability to lock in of our intellectual property in such programs. This includes for example the assignment to thousands of pictures of white balance values and other low level editing metadata, but it could also cover higher level metadata such as editing steps that are much more program-specific.
Users of metadata driven editors like Lightroom and Aperture should shout as loudly when we see encrypted or locked in editing metadata (IIRC the keywords hierarchy info in LR's XMP files) as we were about Nikon encrypting the WB. We also need open standards like we have with IPTC and XMP metadata. I may not know the meaning of the value you've put in that extensible XMP field, but I should be able to read it and have a fair chance to work it out and take at least some of my editing metadata elsewhere.
With metadata, whether it's informational or editing, you've always got to look at your future exit strategy. If you can't see a way to get just your keywords out of a program, should you ever have put them in it? How far should the same stricture apply to editing metadata?