There are multiple occasions where actual versions of source image would be valuable.
First use case is when importing an image database from a third party. e.g. converting from LightRoom or Capture 1. In this situation, it would be handy to have the source image, and the final rendition of the image from the original editor. They logically are a single image, but have two versions which should be managed as a single image.
The second use case is when you need to leverage an external editor. Such as PhotoShop or Gimp. In such cases, it would be handy to generate a TIFF file from DT, send it to the third party application to manipulate. This would allow the user to again manage the single image through the version life.
One possible implementation uses smart file names (There may be other and better solutions). If the original image name was XXXX.CR2; Subsequent versions XXXX.1.DNG or XXXX.2.TIF
Personally, I have an aversion to smart file names, but it was the only solution I could think of which keeps DT from needing to understand or manipulate third party applications.