Actually, many people do. Many of the devices you already own convert grid power to 12v or less. Mostly anything with a "wall wart" you know that big clunky plug that blocks the other outlet when you need it is converting 120v AC to 19v or as far down as 1.2v DC power. If you live on a boat or in an RV, chances are you have access to more 12vdc power than 120vac and have already changed many systems to accommodate. Cars have 12vdc systems too, so anything you have ever charged during your morning commute is part of your 12v lifestyle already.
So what does this have to do with being green? 120vac solar systems are expensive and complex. Generally they require a significant infrastructure investment. (this is a pretty broad generalization mind you and the technology is changing everyday.) 12vdc systems are much more accessible and safer to work with for a hobbyist. I can imagine making a 12v wind generator with no more than a trip to the junk yard and a stop off at a hardware store on the way back.
With a 12v system deployed, a secondary lighting system for your house using modern LED technology could add much more light to your surroundings, or power a stereo system based on off the shelf car stereo equipment. A well designed house could even use 12v fans to move air from different areas effectively making power hungry climate control a thing of the past.
The only real limit is your imagination and resourcefulness.