I think that a blanket statement of calcium is good or bad is a wrong type of statement. Can anyone say that all tubs has a ss heater, or won't be affected by low calcium - I would think the answer is no.
Repeat_Offender's tub manufacturer recommends it, Taylor recommends it for balancing purposes and to keep foaming down but that doesn't mean that everyone has to or should add it.
My opinion is that I add it for the water balancing purposes. I have read that heater could be affected but have no proof. It is very probable that a heater is erroded by low PH. We have people on here that haven't used calcium and apparently are fine, I use calcium ... it's
MY N2 cartridge!