One more usecase - which I admit is niche - is that I want a way to run multiple OSs as transparently as possible. A microvm that boots freebsd in less than a second and that acts almost like a native container would be excellent for certain development work.

Edit: Actually it's not just cross-OS work in the freebsd/linux sense; it would also be nice for doing kernel dev work. Edit my linux kernel module, compile, and then spawn a VM that boots and starts running tests in seconds.