Yes, I asked this myself. I believe the dolomite acts like a buffer to keep the pH from going too low. As it slowly dissolves in the acidic water the plant uptakes the calcium. If I recall my long ago plant classes, calcium uptake is restricted when the pH is too high.
Ultimately it should mimic conditions where some of these multiflorals are found: limestone cliffs growing in pockets of decomposing leaf/plant litter.
As mentioned, it does depend on the species and media being used. I personally would focus on getting the pH right and later try the dolomite (or dolokal) if calcium deficiencies are suspected. Calcium is essential to plant cell walls as one of the first line of defense against pathogens. Often you see erwinia when you have strong vigorous leaf growth (ie too much fertilizer or optimal spring conditions). Since the calcium is less mobile and slower to reach the new parts of the plant, it is more susceptible to pathogens.
Given Theo's infected plants are the more vigorous hybrids so far and not the sanderianums, I suspect this might be why he is seeing the erwinia problems. But just a theory with assumptions.
