The idea is that we provide a roughly command line equivalent interface to VMs as for containers, so that it really is as easy to invoke a VM as it already is to invoke a container, supporting both boot from DDIs and boot from directories.
And one thing in particular we have a focus on here: that the integration points that make it so nice to use nspawn are also available in vmspawn, wherever we can possibly do that. i.e. credentials, sd_notify, and all that other stuff.