Why should a sane person consider building an off-grid or grid-intertie power system ? There are several reasons. To be honest, there is no good financial reason to go off grid if you have grid power available. In this case grid-intertie makes the most sense. But the cost of an alternate energy system, roughly $15,000 to $75, takes a long time to pay off compared to grid power. The cost of running traditional power lines to property can often exceed $50,000 for a short distance.
For some of us, we go off grid because there is no grid where we want to live. So going off grid is not a financial decision at all, it's a lifestyle one. If you want to live in any area far from everything else with any convieniences of modern life (like light bulbs or a stereo), you need a way to produce your own power. There is something really fun about having power, when a blizzard has crippled the entire area, and the power is out everywhere else for days. (which happens alot in this part of Colorado)
For those with grid power available, an alternate energy system can make your bills go down, which is good in states where power is more expensive, like California. I know more than a few people than have brought their power bills down to zero, and some have made the meter spin backwards. The initial cost may be high. but with any tax breaks (like in California), the total cost is reasonable.
There is also the environmental cost of traditional power sources. Nuclear, burning coal or natural gas, or hydro. Other than hydro, which does have an impact on the fish in the rivers, the other source require environmentally unsound practices. So going off grid or grid-interties reduces by that by one little household's worth of power the damage needed to have produced that power by traditional means. Some people build photovoltaic power systems and feed power into the grid gurilla style, just to help reduce our dependance on coal or natural gas or nuclear power.
I also think that it's important for our own personal freedoms to break our dependency on the energy monoply. We all benefit from being able to make our own choices.