Adding one will slow it down, making it easier to fly. Tails work with any kite and inflate with the wind to look great in the sky. A kite without a center spine makes it durable and crash resistant because there is no danger of the sail tearing or breaking in a hard crash. Multiple kite packs pull harder than singles, but are just as easy to fly. Don't forget the thrill factor when you select your sport kite - small kites can fly well, but can't change the laws of physics.
Big kites pull harder, make more noise and are more exciting to fly.