I personally think that pH is the most important parameter to protect the copper heat exchangers but until there is definitive answers one way or the other I see no harm in maintianing a calcium level in the water. Also most portable spas are acrylic and fiberglass and there seems to be some conflicting info on the need for calcium to protect the gelcoat finsish and help prevent staining. Bottom line is that many manufacturers recommend a certain level for their warrenty and THAT should be followed!
Calcium certanly IS needed for plaster or tiled spas and for now I guess we should follow the recommeded guidelines of 200-400 ppm.
For reasons I don't fully understand bromine santized spas have a tendendy to become acidic. Don't know if this is from the bromine itself or the widespread use of MPS in spas. Richard, perhaps you can shed some light on this.

My only real experinece with indoor spas on chlorine was the one my brother had in his old house in Miami. He needed to check his chlorine levels daily since they did not hold. He kept the spa at 104 degrees and used both liquid chorine and dichlor. His other water chemistry parameters tended to stay fairly stable except pH when he used dichlor. It would drop a bit and he would add sodium bicarbonate to raise it. His TA would raise somewhat but the spa had a waterfall that was always on when the pump was running so (I am guessing) the constant aeration would cause his TA to drop over time. I do know that the spa was in a very large bathroom of his house (there was also a sauna in this bathroom!) and there was a constant chlorine smell in the air in this room. Smelled like an indoor pool!