Point number one is that poo contains plant nutrients. This is obvious to anyone with a yard and a dog. Also, sewage sludge can be processed into a fine commercial fertilizer, as you may know if you have ever bought Milorganite.
Point number two is that our current industrial agriculture depends on synthesizing plant nutrients out of oil and gas, to the point where it requires 5 to 50 calories of fossil fuel energy for every calorie of food energy produced, and that's not even counting tractor fuel. (This correspondent has not seen hard data on this factoid but has seen it quoted in multiple places.) This means that all the gains of the Green Revolution may be at risk, as fuel availability declines, which in turn makes all forms of dung-derived fertilizer start to look like valuable resources, including the human kind.
The very very tricky part, is to capture these nutrients without coming down with a lot of flies and disease. Sanitation, that is, the rapid and efficient separation of poo from people, is one of the great advances from the 19th century and before, and is not to be taken lightly. All of this flushing "away" however, throws out the baby with the bath-water.
As this correspondent observes, what sustainability mavens are trying to do, is to capture the plant nutrient value in human waste, while reducing the water-intensiveness and water pollution, and maintaining the health safety. This is best achieved by some kind of relatively dry composting. Because composting takes a while, there is kind of a large volume of stuff "in process" at any given time. As most existing homes do not have provision for large bins under the toilets, the aforesaid sustainnibals (a word I just made up) end up collecting little batches of "pre-compost" in toilet-sized buckets, and dragging them off to the main bin one-by-one. If this still sounds ridiculous, well, I am in total agreement. Sawdust bucket toilets are the most primitive, manual method of humanure composting. There are some better mousetraps out there, and we could use more options.
Draft Postscript: There appear to be two reasons for the water intensity of current technology. One is convenience of transporting the waste - its much more difficult to move around in solid form. The other is historical, having to do with modern sewage treatment's origins in London, a place with rain, the river Thames, and the Atlantic Ocean to serve as an "away".