(It's one of those old wives tales. Hair only seems to grow faster if you have it cut short, because any growth is simply more noticeable when your hair is short than when it is long. In addition, when your hair is short it tends to look healthier and fuller and it doesn't break as much as long hair.
That said, I prefer keeping my hair shoulder length, because it means I don't have to go to the hairdressers as often when it is longer.)