Water certainly mucks up the picture. Local water, a larger source, well water, etc. I had some conversations with people who rely on water for their businesses (one was a print shop and another was a welding and metal place). These guys were telling me that a consistent water composition was critical to their business (for whatever the reason) and large swings in the water composition would cause problems with their equipment. They had controls in place to make small adjustments but the consistency of the water was responsible for things running smoothly. As homebrewers, I assume our water dictates how much focus we need to have on various water ions, etc. Some of us can pay less attention to that while others have to spend more time on it. On one hand, I don't want to guess about what's in the water. I want to be relatively sure that the water I'm about to use for a certain style is close or right on with what I think it is. OTOH, I don't want to turn into a water expert on every single batch that I make. I don't have the tools for that. If things like calcium, sulfate, chloride or bicarbonate had huge swings, I would be screwed. Also, I assume that the source water we all have makes a big difference when an RO system is running... the softer the water, the more consistent the results and maybe the lifespan of the filters. The more schputz you have in the water, the weirder things can get.