Originally Posted by Bekologist
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.
The problem with tread is that it squirms. When it squirms, it becomes a lot more likely that it will lose traction especially in a turn. Those tires with tread on the side are NOT what you want for wet weather street cornering. You want as much rubber against the road as possible (within reason of course) and you want a solid cornering feel. Tread squirming reduces this solid feel and it reduces the amount of rubber touching the pavement (given the same size treaded and non-treaded tires). Because bike tires are skinny enough to not have issues with hydroplaning, a slick tire is your best bet in the rain. Tread helps in the mud becuase the knobs protrude into the mud and grab more mud than a slick would.