The grey water is actually meant to be cleaner. Given the option of that over no water, I know what I would take all the time. Seems some people are going for the no water option. Desaltenation is just stupid, it costs far far too much.
Adding to that, back when we got our house (1990) we built it new, and moved up from jimboomba (no town water at all) so we knew the importance of saving water. We asked the counsil if we could have tank/s. They said NO its a mosquito hazzard... So stuff them.
Greenhouse is a funny one. There have been a few studies over the years. A car is better for the environment than public transport. (in terms of greenhouse gas that comes out of the exaust) its that simple. It wasnt a simple measure of car = x bus = y. It was z people in car, and q people in bus, / emmissions.