I just tried to extract embedded raw files from 889 raw files from a Canon 5D3 having been converted by the DNG converter 6.7 beta - the DNG converter only succeeded with extracting three of them. I also tried with the 6.6 stable version with the same result.
If DNG converter can't easily retrieve the embedded raw files then I think this "feature" should be left out - it's misleading people into taking unwanted risks when you're better off just keeping the originals (it's a bit more messy to organise and archive two sets of files, but I'd rather do that than having to mess around with this)
So stay away from this "feature" unless you want to paint yourself into a corner.
Download and test the most recent version (final GM) of DNG Converter 6.7 and retest...might also be useful to provide a bit more info such as OS & CPU...and also whether or not the DNGs behave normally in ACR or LR. Basic troubleshooting info to test or duplicate your findings would be useful (required) before you can honestly be making such "blanket" statements about DNG Converter...