For me, it comes down to consistency. You can always use chemical water conditioners to take out chlorine and make ammonia harmless and bind up heavy metals, but you still have water that has a bunch of chemicals in them, and possibly nitrates and phosphates. Those things *may* cause problems down the road. Not saying they will... but there will always be that thing in the back of your mind if/when a problem comes up that is saying, "Gee... I wonder if this has anything to do with my tap water."
water, the water you're starting out with is pure. The only thing in the water after you mix it with your salt mix is what was in the salt mix. And it's that way every time you make up a batch of water.
Tap water varies from week to week, season to season. It is anything but consistent.