Dapples can also be a sign of "sootiness" or counter shading on a horse. Like a bay, buckskin, or palomino that is "sooty" will have dapples just as a feature of the coat. And you can have a handsome horse of another color, like say for instance a blood bay, and he can be shiny, healthy, etc, but may not really show dapples because of his color.
I think two things cause dapples. I think the real obvious ones are caused by the color of the horse (for instance grays and horses with the sooty factor) and then a healthy coat will often get a bit of a texture with dapples. Those are harder to see and you often have to stand at an angle to see the dappling. I think those are from good health.
Of course, this is just my personal theory.