I think you just get a different look to your pictures on a gray day. It forces you to shoot differently and choose different types of subjects. There is always local contrast that can be played with and you leave out the sky in a lot more occasions. Gray days make for some moody pictures. And on top of that what is the "right" light anyway? It will be different for different people. If you shoot color certain colors totally stand out on cloudy days. At least they do for me.