Forgive my extreme enthusiasm for this idea, but the beauty of it just keeps coming back to me.
Here are the advantages I see:
1. We can use MC to organize our images. Mc will use the embedded jpegs so building thumbnails, serving to remote clients, and scrolling through the list would all be extremely snappy.
2. Converting raw files to dng is a one step process in Adobe bridge (and I imagine in lightroom too) so there is very little headache in moving between programs.
3. You could move a bunch of images to a temporary folder called something like "Processing" and then open that folder in bridge/lightroom to do all of you color, cropping etc. Then by updating thumbnails in MC you have total synchronisation between it and Bridge/lightroom.
4. You would be able to upload photos to flickr etc from MC knowing that no more work needs to be done on them. MC would just upload the embedded jpeg.
(Currently, dcraw processing is just not good enough, and so you always have to export jpegs from MC, then work on them in some other software, and then decide whether to import the new files back to MC and stack, or just upload to flickr outside of MC and then throw away the processed files, sticking with the original raw files in MC)
5. xmp data would be inside the file like with jpegs. So no sidecar files to worry about.
