I've asked that question a lot, and I've gotten a variety of answers.
There's a lot of nice resources on the Moon and the energy requirements to lift them from the Moon is a lot less than required to lift them from the Earth. There's water, which can be used for propellant, and there's metals, both in the lunar regolith and in more significant deposits from asteroid impacts.. but I disagree that we need humans to retrieve those resources.
The Apollo program only scratched the surface (literally) on lunar science. There's a whole lot of mysteries that lunar geologists don't have good answers for: nonmare domes, rilles, potentially active volcanic vents, the entire far side, and permanently shadowed craters.. but I again disagree that we need humans to do that science, and if NASA was at all serious about it they would have sent at least one rover to the Moon ever.
I have advocated the viewpoint that what should motivate human spaceflight is preparation for living in space. As Jeff Greason said:
You don't learn to live on other planets with robots. Space holds the future homes for humanity - we're going to live there some day if we are going to be a long term surviving civilization.
And by "live there" he means the kind of life we have here on Earth. To me, that means families.
So do I think humans will ever live on the Moon? No, not really. I agree with Dr Jim Logan that most likely there isn't sufficient gravity for humans to safely reproduce on the Moon.. but we don't know this, and he would be the first to tell you that.
Jeffrey Alberts of Indiana University has submitted a white paper to the upcoming Decadal Survey on Biological and Physical Sciences in Space, recommending NASA "reinstate its commitment to
studying mammalian development in space and in altered gravitational environments", including, "the role of gravity at all stages of the life cycle". He is advocating a variable-gravity centrifuge in LEO, as NASA has been directed to do in the NASA Authorization Bill For 2010.
That's all well and good, and it's certainly the cheapest and most efficient way to do it, right now, but should this module be canceled, like the Japanese module was, we will still need answers to this question. Simply, there is no point developing the technology to land on Mars if we can't eventually live there. We'll need to look at other destinations, such as Venus or asteroids.
Whether we have an answer to this question or not, the most reasonable interpretation of the Flexible Path calls for a human lunar return, after flights to interplanetary space, such as asteroids or the moons of Mars, and before a human mission to the surface of Mars itself. Some significant stay on the Moon is a likely goal, especially since we will have solutions to the long duration radiation exposure problem by then.
During that stay, a significant biomedical human factors study should be made, and most likely will be. Actual attempts at studying "all stages of the life cycle", although I expect would be fun if done with humans, will probably have to be done with other mammals. But without the human baseline of physiological reaction to long term exposure of lunar gravity, that data may be useless.