In The Case for Mars, Zubrin argues that lunar bases would be both uncessary for and harmful to a manned Mars program.

To attempt to sum up his argument in one paragraph: It takes less fuel to go from LEO to Mars than it does to go from LEO to the lunar surface, so the Moon would make a poor refueling base. Lunar and Martian environments are too dissimilar to test Martian hardware at Lunar bases. Funding to support lunar missions would cut into funding to support Mars missions.

I know it would never happen, but I'd like to see active lunar and Martian missions. The same boosters and launch infrastructures could be designed and built for both, and maybe we'd build enough boosters to see economies of scale start to kick in. The moon will be a good base of operations for miners, lunar researchers, and radio astronomers. Mars will be a good place to try to set up a self-sufficient base, experiment with farming on other worlds, and search for evidence of life. Plus it would prove to the world that Americans are perfectly capable of doing two things at once.