The beauty of DNG, Alan, is that we don't need to worry too much about issues such as storing information such as Lightroom collections. That's of concern only to Lightroom users, but the wider point is that DNG can store any such information inside the file (would you want to write it into a NEF or CR2?), and metadata that's embedded is usually much easier to move to other apps.
Your point about the time storing XMP files versus backing up DNGs is important and is what I'm driving at. When you get back from Italy, backing up a NEF and its XMP will be no different to backing up a DNG at its creation. But after that, when you make further changes to the picture, there's no point repeatedly backing up the DNG.