I've been thinking about this issue a bunch lately. For years I've had RAW files converted to DNG as Lightroom imports them. I figured that the open source nature of DNG was helpful for long term viability of the files (and it probably is).
But, within the past few months I've started using Capture One Pro 8 to process some of my images. Unfortunately, C1 can't recognize the DNG files created during the Lightroom import process. Maybe I'm doing something wrong, and I still need to do more research to see if there is a workaround. But, when I try to import DNGs into C1, they just never show up.
Overall, this has changed my view on Lightroom's "Copy as DNG" import option. Now I'm just importing the original RAW so I can use LR or C1.
Just my experience and two cents worth.
Cheers,
Jeff