1) When you multiply 3 to 5 gallons saved every time a toilet is flushed, times maybe 1 billion flushes PER DAY, you are talking about a lot of water. Same idea as applied to aerators.
2) Just like with energy saving electric appliances, the motivation is not that they want to save you money on your elecric bill.....every KW saved multiplied by millions of households, means several new power plants that do not have to be built, with the accompanying pollution. Less water down the drain means new sewer plants that do not have to be built.
3) Regarding global warming, show me how that helps local water supplies. New England just had 25 inches of rain in a week. Not good, and it all went down the rivers. True the reservoirs are probably topped off, but come first hot spell this summer, water will again be in short supply. Most of California is in a perpetual drought. Warmer winters make it worse, because one of our main sources of water is the gradual runoff from the slow melt of deep winter snows. No snow, no water.
4) Back to the toilet: Since the toilet is designed to flush with 1.6 gallons, adding more water does not create a "more powerful" flush. It may cause a double or triple flush, but if the first on was a clog, then the second one will just cause the bowl to fill up and possibly overflow. You can observe this by simply holding the handle down.