"The economic development of the solar system, beginning at the Moon and extending on from there addresses the critical national and even global problem of energy
and resources, while providing the opportunity for mankind to incorporate the vast riches that await us there. Any other reasoning for the exploration of space is a hollow effort who's prestige value is no more than adding an additional footnote in some future civilization's history book.

We know that the resources are out there, on the Moon and beyond. Dr. John Lewis in his book "Mining the Sky" has pointed out the trillions of dollars worth of platinum group metals that we know are there, based upon measurements of metal meteorites that have struck the Earth. How does this address a critical national problem?
....
Fuel cells and hydrogen, whether derived from hydrocarbons, solar energy, or nuclear power, is the ultimate future of transportation fuel. Without reasonably priced transportation fuel it is impossible to sustain our global civilization. Fuel cells are much more fuel efficient than internal combustion engines and their exhaust is water, eliminating the perceived climate consequences of continuing to use fossil fuels. The ability to move about at will with humans and robotic systems in the inner solar system is an imperative that brings us these riches. A pretty compelling reason. "

http://www.spaceref.com/news/viewnews.html?id=1315
Linked from http://www.transterrestrial.com/ Why?

I would say the compelling reason is that space could provide cheap and abundant energy.

We have enough energy on Earth, but we don't have enough cheap energy.
On earth we have lots of hydrocarbon- coal, oil, oceanic methane hydrides. And we also have
large potential in terms of nuclear fission and maybe fusion. But they are large in the sense of thousands maybe tens of thousand years worth energy.
But in terms of cheap and abundant energy Earth has very small amount compared to what is available
in Space.

The obvious source of cheap and abundant energy in space is the Sun- a unbelievably massive fusion reactor which will operate for millions to billions of years.
The problem with solar energy on earth is it's diffused- it's not concentrated form of energy. Even if solar panels [or any solar energy collector] were 100% efficient they still would require acres of them to collect significant amounts of energy. Now, there is a lot of space available on earth and something as small as the state of Texas or less than .1 % of Earth surface area might generate enough energy- or worse case if we used 1% of earth surface area for solar collector we could generate enough energy. But even though compared to the entire earth surface it is relative a small area- it's nevertheless a enormous dedication of land [or ocean] area for energy production. It's of same scale as the area of all urban areas of the World. Giving up a chunk of area the size Texas is a significant cost.
Wind is similarly diffused. But if we were willing to give up large areas for solar and/or wind this at the moment is not cheap energy, nor can it be expected to be cheap.

In the space environment one has more area available and the solar energy is more concentrated than on Earth. The sunniest and best locations of Earth are about 8 kw/h per day per year. In space at GEO or Earth/Sun L-1 one has about 33 kw/h per day per year [about 4 times the energy density]. And if you go to Venus or Mercury distance you getting 2 to 6 times, respectively, more energy than at earth distance. At Mars distance which receives 60% the solar energy than at earth distance would receive almost twice the amount energy [14.4 kw/h per day per year] as on the best locations on the earth surface.

Now, on earth the cheapest way to ship anything long distance is by ships- ocean, river, canal, lake transportation. Space is a vast ocean. Space will be the cheapest way to ship things long distance. For purposes transportation, the ocean of space is far more efficient than earth's ocean. So, on Earth might pay $50 per ton using ocean transport. In Space, it could be 50 cents per ton.
So, it's possible that shipping something from Mercury to earth, LA California, then compared to shipping from Portland, Oregon to LA, California.

Today, Aluminum is made where there is the cheapest energy- the cost of making Aluminum is all about the electrical cost to do this [20 to 40% of entire cost- "The most modern smelters achieve approximately 12.8 kW·h/kg (46.1 MJ/kg)"
http://en.wikipedia.org/wiki/Aluminium
About 30 million tonnes Aluminum is produced globally per year, so this amount was produced by most efficient smelters it would be 30 billion kg times 12.8 kW·h/kg. Other source say Aluminum production uses about 3% of the global electrical energy.

One could situation where say 100 billion kgs per year of aluminum is produced in space and shipped to earth surface and not require any production of aluminum on earth [though Earthlings could recycle aluminum] and have aluminum at around 1/2 or less of today costs. With aluminum at lower costs one would use more aluminum and this would give more efficiency than compared to using steel- so having this 100 billion kgs could amount to about 10% of electrical use that otherwise be needed.

Couple this with a space elevator that could say recover 1/2 of the orbital energy and convert it to electrical power. The orbital energy from GEO is about 10 km/sec- which equal to 50 million joules per kg- and 50% is 25 millions watts [6.9 kw/h per kg]. If electrical power is worth about 1 cent per kw/h then each tonne shipped from space to earth produces about $69 of energy. So the economics of space elevator could little to do with carrying up payload from earth, but rather how tonnes per hour could bring down to earth. To deliver 100 billion kgs per year requires over 11,000 tonnes per hour or over 30 tons per 10 seconds. If a space elevator could do such volume it would make $6.9 billion per year if sold the power at 1 cent per kw/h and probably charge a couple billion dollars per year carry the electrical power via cables from space to earth. And instead of solid ingots of aluminum, one have aluminum containers being somewhere around 10% of the mass with some other ballast, cargo, or passengers.
Probably, the only way you get such volume is if the payloads traveled at fast speed- and were loaded and unloaded quickly.

So, there is solar power and orbital energy one could get from space. One could also generate electrical power from nuclear energy in space. The advantage nuclear energy in space is one doesn't the problem related to nuclear waste. Though it may be that if solar energy is cheap in the inner system, and being more expensive in outer system, that nuclear energy will be used more in areas which sunlight is weak- Mars surface, asteroid belt and beyond.

Now, an argument against the future need of energy, is that nanotechnology may be develop so that less energy is needed for manufacturing products and products will be superior so less material is needed, substances have greater durability, and need less energy to operate- whether it's vehicles or computers.
But even if this is true, an important aspect of having a goal getting energy from space is present perception- which is assume we will need a lot of energy in the future. And therefore this perception of problems of having enough energy in the future, could cause actions to be taken- wars, continued poverty, and energy hoarding. Oil at $140 barrel was mostly due to fears of future energy shortage- that hysteria was costly in a number of ways. It could a main cause current recession and could lead to further problems yet to unfold. Thus far, it seems reasonable that this hysteria probably cost about 1 trillion dollars and within next decade or so there could similar panics would could cost trillions of dollars.
So, it seems that putting this off to some time in the future could be quite costly, even if new technology were to be developed in the future which allowed for higher energy efficiency.