Comparitively speaking, a Conti town and country, TT or a shwalbe marathon has a light tread compared to a typical 26" mtb tire.
If someone can find me flawless blacktop, I'd ride slicks, but a tire with tread will grab any rugosity and add traction. Bolts on steel plates, or drain grates, for instance. Also, every expansion joint you ride over. A little bit of tread just makes plain good sense, esp in the imperfect world of urban cycling.