Originally Posted by McDave
Uni-directional treads are designed for maximum traction in one direction only. A road tire with few tread grooves on dry pavement wouldn't matter much, but in the wet the difference could be significant as the grooves are there to shed the water out from under the tire.
This is false, your bike never travels fast enough for hydroplaning to become an issue. The "treads" on a road tire are decorative issue only. A slick tire will provide the most grip in both wet and dry conditions on paved surfaces. Automotive dogma does not apply here.
It's a wonder why tire manufacturers feel the need to put any treads on tires at all. I guess its for the general public who think the exact thing as above ^. I would love to see all slick road tires become more prevalent then decoration tires.
http://www.sheldonbrown.com/tires.html#hydroplaning
http://yarchive.net/bike/tire_directional_tread.html