Uh... try some at-the-limit sliding cornering with the slick vs. knobby in front. Which do you think would be smoother? Which do you think would be sketchy?
As far as traction goes, slicks have better traction on pavement.
I currently use a 1.5 Geax slick on front with a Ritchey 1.00 slick driving the rear. Its a tremendous speed difference over running 1 or both knobbies.
I rarely use that mt bike anymore for commuting. Now it only gets used on rainy days. Most of the time I ride my road bike and it sports 700 x 23 slicks. They reliably carry me over the 2 miles of gravel that I must cross on my daily commute.