As I understand SmartOS was designated to be a host for VMs, for that reason Joyent ported KVM to Solaris and made it so it can be booted from USB drive. So for

As I understand SmartOS was designated to be a host for VMs, for that reason Joyent ported KVM to Solaris and made it so it can be booted from USB drive. So for example updating system is as simple as loading new version on USB drive and rebooting the system.