I think everyone got it a little right.
Like Dave said, these things evolve for some kind of a purpose that increases survival, so the feature is passed on to the next generation.
In the case of threadfin shad, and almost all schooling "baitfish" with false eye spots, the spot creates a false image, making the school appear larger and more confusing to focus an attack on. So, in that case, the spot is for survival from attack in a group.
In the case of the Bream family, and other fish that don't school tightly, but do share areas as groups, it's probably a sexual display for mating. Ever notice how fishes colors get fired up during mating season?
In the case of trout, which are basically a predator, the spots are for camouflage, to let them blend in with their surroundings when they're on the hunt. Same with the bars and patterns on bass, and the white bottom/dark top on almost all predator fish.
There are lots of salt water reef fish that are brightly colored to advertise that they are poisonous. Or really horny!
Anyway, I just try to simulate the color scheme of the local preyfish, or whatever I'm trying to imitate with my lure, not match it exactly. That kind of paint job I leave to the guys here who can really paint.
My lures, even the floaters, are basically reaction baits. If the lure has a shape and size that's close to the prey, and a decent action and color scheme, it will be eaten.
Fish aren't geniuses. Fishermen are far more picky than any bass.
Well I think we can conclude that kill spots represent different meanings on different fish.
:?My prediction for the spot being on the back of a fish would try and get its predetors to attack from the rear. If the fish has spines (like the fingermark bream) a posterior attack could easily be lethal for a predator (kind of like choking). Ever seen a dead bass w/ a bluegill stuck in its throat? they are normally being consumed tail first.
As for the poor threadfin having a kill spot more toward the head, it has no spines to prevent a rear attack. So a head attack is just putting itself out of its misery. Bon appétit mr. bass...