What's the matter with Arkansas?
A while back the legislature of the great state of Arkansas decreed that the possessive of the state's name should be "Arkansas's." This caused a few grammarians to go ballistic at the time, but in line with your "it's the sound not the letter which should determine it" dictum from the "a historic" vs. "an historic" debate, I'm guessing you'd be okay with their decision.
When this ruling came down (or up?) from Arkansas in March of last year, I was so incensed that I couldn't even bring myself to write about it. But if now, almost a year and a half later, my arguments are being misappropriated to defend something as egregious as a superfluous S in the Arkansas possessive, a remedy is definitely needed.
The late great folksinger Utah Phillips said, "The state can't give you freedom, and the state can't take it away. Freedom is something you're born with." Same deal with punctuation. The state can no more legislate grammar that it can legislate your soul.
The possessive of "Arkansas" no more needs another S than the possessive of any other word ending in S--which is to say, never. It's a matter not of pronunciation, but of punctuational economy.
I know some schoolteacher once told you that some words that end in S get another S after the apostrophe. But do you remember which ones? Yeah, I don't either.
Fact is, there are so many conflicting directives out there that all the "rules" have been rendered meaningless. So it's time to throw them out and start fresh with a rule that's simple, economical and universal: If it ends in S, just slap on an apostrophe and be done with it. Don't gum up the works with another letter.
Interestingly, this letter writer takes the opposite tack: He goes on in his note to say that he always writes "s's," no matter the circumstance.
Admirable in its obstinacy, but wrong. Don't waste the ink. It's almost as wasteful as using taxpayer bucks to legislate punctuation.