Not entirely correct -
High pH means that any ammonia in water will tend to remain as such while lowering pH converts it to the much less toxic NH4+ form (protonated).
pH stability is also of limited importance - instability can be OK if you know the cause of it. Planted tanks w/ timed, pressurized CO2 injection, for example, can see pH swings of 0.5 unit every 12 hours, this is inconsequential to the fish. If, however, your pH swings randomly due to lack of GH (low/no buffering capacity), this, coupled with general osmotic stress, can be bad for your fish.
Adding buffers to the water to control the pH is generally advised against because people generally don't want to be bothered with trying to match their water for water changes to what's already in the tank. If you just dump a bunch of buffer into the tank and then do large PWCs with tap water, you will have issues. If you take the time and trouble to match your incoming water with what's in the tank with respect to GH, KH, pH - the fish won't notice the change except that the water is cleaner. To do this well, you need to know how much of what is in every gallon of tank water and you have to be willing to do the math at least once so you can get it right during PWCs. If you do this sort of thing, it's best to replace evaporated water w/ RO so you don't inadvertently allow anything to build up over time.
While I have no idea why your barb died (gravid stress, dropsy, combination of both?), I hope this clarifies a bit of the reasoning behind some of the "not-entirely-true" rules of thumb so often passed on.
