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Originally Posted by dcb
And wide tires are not all that good for on-road traction.
If you have ever ridden a 10 speed AND a mountain bike through the slush you will know exactly what I am talking about. The skinny 10 speed tire cuts through the slush and on to the pavement, displacing only enough material to reach the road surface, leaving the rest of your weight for traction. The wider tires do not penetrate as well, and you are sliding around on the surface of the slush.
If a narrow tire and wide tire have the same tread pattern then the narrow one will still resist hydroplaning better.
Where wider tires help is in racecars where you are targeting several Gs and expect to change to new tires regularly despite spreading the loads across the wider width.
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The big difference between skinny/wide bicycle tires, is that different sized car tires inflate to the same pressure whereas the different sized bicycle tires can vary by 60+psi.
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Wider tires also help when there are significant chunks of road missing, so that some part of your tire is more likely to be in contact with the road even if half the tire is over a pothole, or in off-road situations where your forward thrust is part newtonian 
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I hope you are not using newtonian on any of my trails. You might get a lecture about not tearing up the trails for the next guy. (ok so we don't have much in the way of mud) The most impressive run I saw as a little truggy with a 4cyl engine and lots of gearing putter up a waterfall without hardly slipping at all.
As far as wider/skinnier tires, I think it depends a lot on car weight, my wifes 05 Accord bulges the front tires even at max sidewall, and we went wider when we changed them. My truck tires now stay round even when I have a heavy load in the back. Unloaded they will stay round down to 25psi.
So all in all my answer is it depends on the vehicle, weight, typical driving conditions... etc as to weather wider is better or not.