They ostensibly help with traction in the wet, and can help with traction if there is any sort of loose substrate on the surface (which is most of the time). Can you think of a reason not to have them, other than some impossible to measure decrease in rolling resistance? The GPs are lighter and better made than the rubinos, so I'd not get too preoccupied with tread. Do they even make smooth tires? The only 'treadless' tires I have seen just have a fine knurled pattern to them; its still tread. It works as a great tire wear indicator too.