While javascript engines are generally not shipped with OSes these days, it's moot because chef bundles its own ruby and ignores the ancient one in the OS. I suspect that's the case with python-based packages as well. I know HP Server Automation bundles its own python. It's easier to support a consistent python version across umpteen platforms than to debug your code in all the varying versions across so many distros and distro releases. Maybe Fabric can use the OS's python, but that's about the only example I know.