vserver, lxc and openvz were 3 competing projects to add process-level isolation to the linux kernel. We used all 3 of them extensively (the ancestor of docker was based on vserver, then ported to openvz, then finally to lxc). They all had pros and cons, but in the end the only meaningful difference is that lxc got merged upstream, and the others didn't.

> would make sense to modularize the back-end so that "docker" (as in the daemon/management tools) would work for maintaining jails and/or zones as well?

Absolutely. That is the goal, and starting with 0.8 Docker's architecture will be modular enough to support it.

See this blog post for details: http://blog.docker.io/2013/08/getting-to-docker-1-0/