"Marketing indeed, as everything seems to come down to marketing and perception these days. Often, distilleries rely old world methods to insure that the flavor profiles that their customers have grown accustomed to do not change (think New Coke)."
Let's not think New Coke, ugh. I'd agree that although marketing is the preeminent last step to sales, even more important is the takeover of so many formerly independent, small companies by the biggies, the conglomerates for whom "profit" and "volume" are the key words. These companies could care less about established and even successful profiles (think Cruzan Estate Diamond) but focus on reducing costs and increasing sales.
This is exactly why (unlike the single malts) the rum shelves are being taken over by droves of new flavored, and now spiced products. And I mean taken over. At Total Wine, even the top shelves are now displaying some spices, and formerly expensive, non-flavored sipping rums moved lower, and just plain disappearing. So much for old world methods and preserving profiles. Just isn't so.
"Techniques used before major technical advancements were (and still are) often a result of economics. The ability... or the inability to aquire fresh clean potable water can be... and certainly was very difficult for island dwellers.
The use of dunder and repurposing rain water surely was part of the economics of the day. The use of spent barrels for aging the silver. On an island, resources are surely finite."
Although the use of dunder is common in Jamaica, that is not generally true elsewhere. And I'd prefer the descriptor of "redirecting" rather than "repurposing". Cisterns were commonplace, but so were wells and handy streams. Indeed, distilleries were often sited near streams. Barbados is a case in point, being one of the few coral islands in the area. The water that was filtered through the coral was exceptionally pure. Interesting bit of history, but really has little to do with modern, industrialized distilling operations.
"Well today that frugal attitude has evolved into a flavor profile that customers have come to expect."
Frugal and rainwater are antonyms. Today, a rainwater system requires added costs, operating expenses, etc. They require, uh, rain too. Those who use rainwater are seeking purity, not savings.
"BTW, the rain water may also assist in a more speedy ferment time.That further allows the distillers to project more accurate production cycles."
I'd guess you are assuming rainwater is less pure and contains local native yeasts. But let's say you are right - to risk your product on unknown and/or uncontrolled yeasts is highly risky. Most distillers go to great lengths to minimize or eliminate native yeasts, and have spent years finding and buying special yeasts that produce their preferred profiles. In sum, I disagree with this premise entirely. Rainwater, properly harvested and stored should be extremely pure - and will not shorten fermentation times.
Fermentation is just too important to relegate to chance.