http://www.pfmmag.com/JulyAugust2004/Julyauguststuddedtires.htm
The Role of Studs
So where does that leave tire studs? The answer is simple: studs are there to provide additional grip on ice and hard packed snow that becomes like ice. If you rarely if ever drive on ice, then you do not need studs. However, because studs provide extra friction by biting into icy surfaces, the tread design can be slanted more toward optimizing performance in deep slush and snow.
Similarly, because some of the grip on icy conditions is taken care of by the studs, the rubber compound can be made harder to provide significantly longer tread life. In other words, by using studs, the designer can produce a tire that can cope with a wider range of driving conditions, such as ice, slush, and deep or hard packed snow, and still provide longer tread life as an added bonus.
http://www.pfmmag.com/Sept Oct 04/septoct2004studdedtires.htm
A complete test series was performed in 2003 by the Finnish magazine Tekniikan Maailma comparing ten studded tires and seven studless winter tires from manufactures such as Goodyear, Uniroyal, Michelin, Nokian, Bridgestone, Continental, and Gislaved. Tests were conducted on smooth and rough ice, hard packed snow, slush, and wet pavement, and the results demonstrated the overwhelming performance supremacy of the studded tires, on ice and slush, while the results were more evenly matched on hard packed snow.
The only test where studless tires were consistently superior was 'noise on dry pavement'. These tests also illustrated the great strides made in winter tire design. The best tires were made in 2002, while the two tires rated lowest overall were one studded and one studless tire that were first introduced in 1996. The average values of the friction values on smooth ice of the 2002 studded and studless tires were 0.23 and 0.17 respectively, and a comparable value for an 'All Season' tire would be about 0.13.