Sure but the majority of water in the U.S. comes out of the tap at 7.0
I'd love a citation on that if you have one; instinctively and anecdotally I don't believe that's anywhere near accurate.
I use R/O water (but have also used tap for grows at this location) and don't need to use Cal Mag (the most overused fert, IMO) because it's supplied by my nutrients.
The money I save in pH down (tap is 115-150 PPMs and 7.5 - 7.8 pH, depending on time of year) more than makes up for the cost of the R/O filter - when using R/O water, I mix my nutes and my pH is perfect, as opposed to adding tons of pH down to the mix after adding nutes.
The fact that the pH buffers are removed from the water so that the pH isn't trying to get back to 7.5+ for the first few days after each reservoir change is priceless to me.
If I ever notice a calcium deficiency, I just use a little bit of tap water to fix it (VERY rare).
But I agree, I can and have grown without my R/O filter and it's more than possible to grow great weed; it just makes my job tougher to do so (IMO).