There are many difficulties with using nuclear fuel on a space mission - either safety or political depending on which camp you hold up in. Mars Direct requires a nuclear reactor to create the return trip fuel within the two years between launches.
To circumvent this requirement, NASA's Reference mission proposes to use in-situ propellant for a Mars launch vehicle, but a pre-fueled Earth Return Vehicle for the leg home. This idea introduces the additional complexity of a orbital rendezvous and makes maintenance of the ERV much more difficult. Plus it adds the expense of a third vehicle and launch to every mission.
My idea, like the NASA Reference Mission, involves a third ship - but that's where the similarity ends. The third ship isn't used for the return trip, but to gather solar power for the ERV. A space based Solar Collector would gather solar power and beam it to the ERV on the surface of Mars via microwave radiation.
In addition to its primary role, the Solar Collector can also function as a relay station for Mars-to-Earth and Mars-to-Mars communication - ensuring that the Marsnauts will always be in contact with each other and with the Earth. The Solar collector can also provide aerial reconnaissance to the Marsnauts - enabling up to the minute reports on both terrain and weather conditions. Finally, the Solar Collector can provide power to the Hab.
This approach is not as expensive as the Reference Mission because the solar generator can be used on multiple missions. Nor does it have the mass or rocketry requirements of the Reference Mission's ERV. It's more expensive than Mars Direct's nuclear reactor - but may be safer.
In summary, a Solar Collect, in orbit around Mars and beaming power to Zubrin's Earth Return Vehicle provides many benefits. No tricky docking, no debatable nuclear launches, better communication capability, better power availability all over Mars, a proof of concept for Earth based power systems, and less expensive then the Reference Mission's approach. On the minus side: more expensive then Zubrin's reactor.