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depends on the knobbles, not all knobblies are bad on tarmac, some have better rolling resistance than slicks of equivalent width, plus they allow you to leave the tarmac and tackle trickier surfaces with greater confidence, and of course they are better for badly damaged tarmac.
If you know that the ride is going to consist of 100% smooth pavement then slicks are best, but if you are riding bad British tier roads and expecting to hit anything rough then some knobblies are ideal.