The way serebii has it worded is incredibly awkward. The Shining Charm increases your chances of encountering a Shiny to 1 in 2370~ instead of the usual rate, or actually triple the normal rate (I guess 33% of whatever denominator the usual Shiny rate is in Gen 5, since it got reduced from previous gens). This apparently also works on breeding, and if using the Masuda Method it becomes 1 in 1024 chance of getting a Shiny hatch.

But yeah, awesome as this is, it takes work to get. Entire National Dex seen? Doable, but at that point you've probably come close to actually completing the National Dex. And it seems to be a Key Item (unlike previous reports), so no fudging around with trades or anything.