There is a lot of truth to what you’re saying. There often is not a great reason to keep freshwater fish and plants in brackish conditions, even if they are salt tolerant (paraphrasing an earlier article I posted). And frankly, with few exceptions, the fish I list as being freshwater tolerant (that is, 1.000-1.0XX) are probably best off kept in freshwater conditions.
On the specific subject of livebearers, currently the only livebearers that I have listed on my page that truly, unarguably need brackish water are the the Foureyes (Anableps spp), the Pike Livebearer (Belonesox belizianus), and River halfbeaks (Zenarchopterus spp). I do agree that the majority of Poecilids (including Platies, Swordtails, and Mosquitofish) as well as the Halfbeaks (Wrestling and Celebes) are perfectly at home in freshwater aquariums. Moderately Hard conditions are perfectly fine too. I would like to point out that the values I give for brackish tolerance on the care sheets ARE fine for long term health, though. I generally don’t list short term tolerances (like how the Mosquitofish can survive in 1.050 for a time, but not long term).
Mollies are another matter, though (and to a degree, Guppies). There are two parts to why brackish conditions are HIGHLY recommended for keeping these naturally freshwater-to-brackish fish. One, Marine salt mix raises the hardness of water. For the majority of folks who have soft or moderately hard water out of the tap (say, up to 12 dH) this is not enough to ensure long term Molly health. Two of the common species of Mollies need at least 15 dH, and the Giant Sailfin Molly needs at least 20 dH. Many people do not have water this hard. The other side is that Mollies are extremely Nitrite and Nitrate intolerant. Salt in water reduces toxicity of Nitrites and Nitrates within the tolerances of Mollies.
Aside of that, brackish conditions fight a number of freshwater illnesses, parasites, and diseases. When kept in freshwater, Mollies are “prone to a number of diseases, including whitespot, fungus, finrot, and ‘the shimmies’” (source). Few, if any, brackish parasites are established in the trade.
The sad truth is that long-term Molly health is a gamble in freshwater. There’s really no reason not to do brackish water (even if it’s just 1.001 or 1.002) with Mollies… Marine Salt mix is cheap and many of the best tankmates for Mollies (that is, ones that can tolerate 15+ dH) are brackish tolerant too.
Phew, sorry for the long post.