Reef crystals with sps

[Aquattro]

The advantage is probably just the elevated levels for Ca/Alk/Mg...maybe some other stuff. It reduces or eliminates the extra dosing required, especially with new make up water. If you dose the tank to Ca of 440, you don't want to do a large water change with IO at 380ppm, so lots of people dose the new water first. RC means you may not have to do that.
If you dose though, or have a good Ca or kalk reactor, there may not be any advantage. But what's the price difference over a year? You might recover that cost from the 4 extra frags you grew....