Trading is something I do nearly all the time. It's a way for me to give people presents.
"Tadah! A breeding reject for you, a breeding reject for you, and you..."
Seriously though, trading is important, especially if you're finishing a Pokedex. Through trading, you can get Pokémon exclusive to the other version of the game, as well as evolve specific Pokémon that need to be traded in order to evolve.