You're absolutely right, you're just exchanging ions, one salt for another. Either way RO water will need to have a Cal-Mag supplement. Soft water is not as friendly to your plants as hard water though. Some places sell an artesian post filter for the RO that replaces trace minerals, might be an option to help out the plants?Do you have LA city water? I do and my ppm is 120 going into the RO and 5 to 10 coming out. The funny thing is that our house has a water softener and the output is 250! I called the water guy and he said the softener removed the calcium that screws up the soaping and rinse ability and replaced it with sodium or potassium depending on which salt you use. You feel the difference even though the water actually has more PPM. That's why soft water and RO water need calcium and magnesium supplements for flowering, or so I am told.