I agree with sasha, there are many circumstances that you might want to go back to the physical disc. Macintosh multimedia discs are a good example. That being said, I also believe if you have a well documented disc imaging process or at least a policy on imaging certain disc formats with a validation step to ensure all sectors copied, then the media could be discarded and the disc image would be the digital master.
We generally will keep BIN/CUE or ISO of:
DVD-Video (preserve menus, interactivity)
HFS/HFS+ filesystems (Resource Forks)
Some Audio Discs with enhanced features, CD-Text