Indeed, but then the saltwater livebearers seem to be used in two specific cases. Firstly to mature tanks. Mollies are nitrite tolerant and good algae eaters, so they're ideal for maturing aquaria. The second use is guppies in micro-reef tanks, where one or two male guppies would be much nicer (and safer) than a damselfish or some other hardy but larger marine.
There are a few brackish fish that do hold their own in big marine, usually fish-only, tanks. The silver scat is one; it's a beautiful fish. The other, oddly enough, is the shark catfish. I've seen public aquaria with Arius spp. in reef tanks. While they will eat shrimps and crabs, they're fine with corals, anemones, starfish, etc. For anyone after a substitute for a shark, they come pretty close.
What's more of a surprise for marine aquarists is that a good many of their fish are brackish water as well. Some of the puffers, sharp-nose puffers, snappers, grunts, damselfish, and batfish, for example. They can't be adapted to freshwater (with a few exceptions) but at SG 1.010-1.015 depending on the species they can be kept in brackish water tanks just fine. The boundary between fresh and salt water fishkeeping is obvious, but in the real world, these habitats blend into one another.
Cheers,
Neale