The GTS is terrible for trading with yourself. You have to exchange a Pokemon, and, although you can just catch some Route 1 Pokemon, that's still extra time, and, if you're offering some rare or more coveted Pokemon for some Pokemon you can easily get on Route 1, it's not unlikely that anybody who stumbles upon your trade is going to take it. You can also only offer one Pokemon at a time, if my memory is correct, so transferring multiple Pokemon would be very tedious and take a lot of time.