Hi Chemgeek,
just only noticed this post today... yeah I guess we both responded on the previous thread about the same time < you owe me a beer...lol >

I don't know enough about chemistry to know why calcium is needed so much in hot tubs. Again, the store would respond that it's needed for the metal parts of the motor.
The shell is acylic... the jets are heavy plastic ( pvc ?) with really thin plastic where they lock-in. They definately get build-up on them. You can take them out & scratch off the build up.
we use a product called tub-clean before each draining that is supposed to remove that stuff. there are other chemicals to prevent scaling as well.

With regards to Bromine: It is used because it stabalizes better ( & disappates less) at higher temperatures than chlorine... ( hence I was wondering since I keep my pool at 90F, if I shouldn't switch to bromine... but then i read Ben's sticky about how it's hard to get rid of it)

The Bromine levels tend to remain high very well on their own ( particularly with a bromine puck floater) & shocking the tub isn't necessary... unless you allow the level to fall down to bellow 1ppm.
Most people keep there tubs between 5 to 7... we like to keep it around 10ppm !
it helps with acne skin conditions that my DH has... & BTW it is awesome for diaper rash for the kids

Calculations are so simple even I can get this:
you generally lose 1 point per bather per 15 minutes
This is the standard, however if your bathers have been swimming in the pool of high chlorine ( say above 3ppm), then they're less dirty & so the loss rate is lessened.

I have one hot tub outside & one inside.
The outside tub's lid cover is very good at keeping everything contained. You don't even see any steam comming from it in the middle of winter... ( I'm talking minus twenty-five & bellow, people! lol)
I don't believe that bromine smells less then chlorine. It smells different for sure, but they both are very strong smelling, both dry & in the water.
My cats seem to get "high" from smelling both chemicals. I would say that bromine seems to make them have a more "mellow" stone & chlorine seems to make them get the "bezums" ( running crazily all over the house, bouncing off the walls)

The bromine is really not that expensive, since not a lot is needed to be added ( regularly, once the levels are up).
It's the "resist" that costs the most. always somewhere between 9 to 14 tbs added each week... I can't remember now if that was PH up or Alk... either way, we got that covered now with our supermarket products & are saving big bucks $$$

Back to the calcium situation...
I would wonder if it's something that is in-part added to help drive the TDS up...?
When the TDS reaches above 1200 & no more than 1500... you must drain the tub & refill. Then you have to start all the chemicals all over again ( & there are several start-up chemicals needed on top of the regular stuff).
Hot Tub stores make their money from chemicals sales.. big time. so adding calcium a lot will cause you to have to buy more start-up stuff more often.
even if you didn't get up to 1200 ppm TDS, they still advise changing out the water every four months.
Personally I like the " Tub Ride " when the TDS is higher ( towards the end of the water's life). Fresh water doesn't bubble as well...lol

I would also be interested in hearing if anyone can answer your question about the necessity of Calcium in Hot Tubs.

Hope I addressed everything in your post... my memory ain't what it used to be...
< I swear kids eat your grey matter...lol >

sincerely,
Hot Tub Hottie
ROTFLMAO!