First of all, shiny Pokemon are not stronger than regular Pokemon, so they wouldn't give anybody an advantage.
Not many people are willing to give up their shiny Pokemon. If the person does not want it, or want something he or she wants something better, then they will be willing to give it up. The person they will trade it to would want it, so the trade is a mutual benefit to both people. Otherwise, finding them in the wild is luck anyway.
By the way, I got a shiny Golbat (now Crobat) from a girl who wanted Clefairy. She wanted it more than Golbat, and I got a shiny Golbat in return.