Yeah, Reburn and Bigsby summed it up well. I know it's called a Calcium Reactor but the most important stability it brings to an SPS system is level alk parameters.
Agreed that a drop in Calcium generally won't cause STN and cause SPS to die but your alk will drop with it. With the 2 dKh of alk being consumed in 24 hrs in my system, being offline for a week as you mentioned would have me coming home to a bunch of pretty white sticks.
I don't test 2-3x a day anymore, that was just for calibration purposes but I still test weekly just to make sure things are inline. The most comfortable I've felt in gaps between alk testing is 2 weeks, when my tank was on automatic for about 2-3 years. Growth rate was consistent as well SPS population.
I test the rest as a formality monthly (Ca, Mg, NO3, PO4, salinity, K, and I2). They typically don't change as I don't do water changes and the CARX seems to take care of keeping Mg stable.