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True. I suppose you could potentially have salty water that was relatively PH neutral. I know that Brewlab in Edinburgh actually have to harden their water slightly.
What interested me recently was the fact that when you adjust water for beer brewing, you actually have to vary it according to how darkly roasted the malts are. I'm fairly sure (as I was suggesting before) that this would apply to coffee roast as well, though in what way I'm not sure.
rodabod-
you could do, but the bigger issue is the dark roast. darker roasts are more soluable and have less compounds to extract, so in theory water with lower mineral content ( which has lower extraction power) would make it easier to avoid over extraction. BUT seeing as its much easier to achieve this by adjusting your dose, time, brew temp its hard to see the benefit.
Matisse
Potentially, but softer is.not always better. The composition is key.