A lot of people get away without RO for hydro, so sure, it may not be "necessary". It likely highly depends on the quality of your pre-filtered water to begin with. If you have some nice 100ppm tap water, you might be just fine with a tall-boy. But if you have 300+ ppm, I think you're gonna have issues without strong filtration. Personally I don't like to always have to constantly adjust ph, I like to set it and forget it. It seems that inevitably with tap water, the ph is always rising due to carbonates in the water. Everyone's tap/well is different. RO is a blank slate which you can count on for consistency. I like consistency.