The Driver Sessions Spec says:

"For unacknowledged writes with an explicit session, drivers SHOULD raise an error. If a driver allows users to provide an explicit session with an unacknowledged write (e.g. for backwards compatibility), the driver MUST NOT send the session ID."

As demonstrated in shell_can_retry_writes.js and perhaps other places, the shell allows w: 0 writes in an explicit session. It therefore risks exactly what the spec wants to prevent: concurrent writes with the same lsid.