In many cases, I don't think it's about being 'green' necessarily... In certain areas, traditional houses are just too expensive for the younger folks that tend to buy into tiny homes. If you can afford it, it's a better short term investment than renting an apartment too; more flexible, more desirable, and more financially sound accommodation in the step(s) between the college/early 20's lifestyle and the early 30's "I've got enough money to buy a house now!" lifestyle.
Personally I've put a lot of thought into tiny house-esc living for the coming years of my life, though I don't think I'd go for a NG stock car as a base... I have enough trouble with hitting my head in coaches, let alone freight cars