"For the guy who drinks 10 liters of water a day in all but the most extreme circumstances, an excess of potassium would be the least of his troubles. Think hyponatremia?"
Using your recommended Morton's Lite, the 3 grams of sodium (plus whatever's in his diet) should take care of that, right?
"In any case, 10 X 350 mg = 3500 mg, still less than the recommended daily intake of K"
The recommended amount for most people, but not including what's in his diet. Others' diets will differ, but quick calcs show that I take in ~ 5-8 grams/day of potassium from food when backpacking.
"One would think such people would be aware enough of their situation to avoid electrolyte supplementation in any form."
One might, but not everyone has particulary good access to medical care, and even among those who do, not all take advantage of it. In some surveys, about a third of people report that they don't regularly or ever see a doctor. People can have significant health changes and not be aware of them.
"Why take the exception and use it to argue that most people should avoid a cheap, simple means of taking in a modest amount of sodium and potassium when exercising for an extended period?"
That was not *at all* my intent. If my comments read that way, then I have expressed myself poorly. Indeed, I'd argue that most people *should* be supplementing with modest amount of sodium and potassium while exercising for extended periods if they're not getting adequate amounts already from their diets.
"The daily intake recommendation for potassium is around 4000 mg, so 350 mg X 3 liters of water, a fairly average amount for most hikers, is hardly "straight up"."
By "straight up," I refer to the supplement itself, which is nearly 50% KCl. Due to the high concentration, its potential for misuse is significantly greater than that of most other dietary potassium sources. If used as you suggest, it will cause no harm to most people. (And indeed I have used it myself that way, during conditions when I wasn't eating real food with plenty of it.)