But isn't libcontainer just using the same mechanisms in the Linux kernel as LXC anyway? It seems a little redundant to me; although I see value in the 'execution driver' approach as I assume they will ultimately deploy Docker on Solaris and use jails there.

I'm annoyed that the new LXC execution driver appears to have broken something in the LXC networking; previously I was able to run a Docker container using LXC networking, give it its own MAC address and have it assigned an IP address by my router and be visible to all other devices on my network (use case: a container to run Plex Media Server).