There is one small advantage, and I'm not saying this makes the price justified...........and that is "some places" the tap water is SO bad you really should not use it in a cooling system. My years in the Navy in San Diego comes to mind.......
My grand mother had a dairy, the water was so hard down there u could almost walk on it. I use tap water here, which is pretty good water, but I have double filters anyway.
I can not see paying for someone else to mix it half and half and charging twice as much for it !