They do, as does Bedrock. Bedrock even takes it further, as you can have packages from different distros/package formats in one system. Although it's a real PITA to install. Been there, done that. With the next release in September things are supposed to get easier there.
Not long ago I publiushed an article in the german magazine Linux User on theserather exotic systems. I was a follow up on an article about Poettering's essay on how to build distributions in the future. He grabbed quite some ideas from these pioneer distros.
greetz
devil