Elon actually addressed this, you only need the booster from Earth. For all interesting targets the spacecraft can launch by itself and get back to Earth. The

Elon actually addressed this, you only need the booster from Earth. For all interesting targets the spacecraft can launch by itself and get back to Earth. The point of the booster for Earth is both to get out of Earth's gravity well and through its atmosphere (both of which are far less of a problem on Mars).