So, our literature qualifies this statement a bit, if the roads where your ride are excessively rough, then a tire with tread features may have some benefits. Hard to know where the tipping point is but where I ride the roads are fairly well maintained and for as long as I can remember I've always used slick or mostly slick tires.

From our website: http://www.schwalbetires.com/tech_info/tire_tread
Why ride a slick tire?

Even in wet conditions, on a normal, smooth road, a slick tire actually provides better grip than a tire with a tread, because the contact area is larger.
The situation is much different on a rough road and even worse on a dirt trail as in these cases the degree of control provided by a slick tire is extremely limited. A slightly serrated surface on the tire tread can have a positive effect on tire grip, as it creates micro interlocking with rough asphalt.