The data is only valuable if you can use it. If you are not using the software provided with the camera maker it doesn't matter if it's maintained or not since you aren't using that data with third-pary software. But, it is being stored in case Canon/Nikon/Etc have a change of heart.
Why would you "burn the bridge" to using software provided by the manufacturer? If Canon or Nikon was coming up with great software to edit your RAWs, would you, by principle, discard this solution? I wouldn't. This has already happened with Canon, to a certain extend. DPP has improved since the first version, and their noise reduction algorythms are put into DPP 2.2 (I believe), which I can use on my 20D AND my 300D (as well as the new Picture Styles, if I wanted to).
On the other points, because I do have DNG converter, if the proprietary RAW is dropped by the manufacturer, I can then convert. By keeping the RAW file, all my options are open; not if I delete them. It's that simple.
As far as all data being present in the DNG file, that may be true (I'm not a specialist), but in that case it still remains in the DNG file as proprietary, ie not understandable if you don't know what it means. As a consequence, the format being open does not mean that all data is readily usable. This is the reason that softawere supporting DNG need to support the original RAW format to support the DNG for this camera. A lot of people (including me), thought that softawre supporting DNG would have to bother developing specific support for new cameras, as long as DNG COnverter supported it, but it proved wrong, for the reason above. DNG does NOT provide a standard RAW format, but a standard file wrapping of the RAW datam to my understanding.