In my 48 years of driving in Minn., Vt., and Mont.,I've used numerous snow tires, studded and unstudded. I'm convinced that studs dramatically increase traction on snow and ice. For about 14 years I've used nothing but studded tires. Numerous tests, especially in Canada and Scandinavian contries, show that studds considerably increase traction. As to decreased traction on dry pavemnt, in my opinion and experience, its a myth. Think about it and look at a studded tire where it touches the pavement. The studs do NOT keep the rubber from touching the pavement. The rubber touches the pavement just as much as with an unstudded tire, so you get just as much traction from the rubber with a studded tire as with an unstudded tire. If you don't believe me, stomp on the brakes on dry pavement with studded tires, or floor the accelerator from a dead stop and lay a patch of rubber. In addition to the scrapes from the studs you'll see just as much black rubber marking on the pavement as you would if the tires were not studded.
There's a difference, by the way, between a true snow tire and an M+S tire. The following is from the Tire Rack website at tirerack.com, which has a wealth of info on tires:
"What's the difference in snow traction between an M+S (Mud and Snow) branded tire, an All-Season tire and a purpose-built winter tire? While many drivers probably aren't absolutely sure, it can be the difference between getting to work, getting home or getting stuck.
The original definition of M+S tires is based on the geometry of the tread design. The M+S designation was first used to differentiate the knobby bias ply tires intended for use on muddy, and/or snow-covered roads from the straight rib tires used on early cars or trucks. Tires with tread designs that meet the definition may be branded with the letters "M" and "S" in several different ways (e.g., M&S, M+S, M/S, MS, etc.) at the discretion of the tire manufacturer.
When early radial ply tires were also found to deliver more snow traction than the straight rib, bias ply tires, the tire companies introduced "All-Season tires." Supported by advertising, All-Season tires have presented an unspoken promise that they, throughout their life, can provide traction for all seasons...through spring's rain, summer's heat, fall's cooling and winter's snow. While this combined offering has made All-Season tires popular, many drivers have learned that a geometric definition doesn't guarantee winter snow and ice traction.
In 1999, The U.S. Rubber Manufacturers Association (RMA) and the Rubber Association of Canada (RAC) agreed on a performance based standard to identify passenger and light truck tires that attain a traction index equal to, or greater than 110 (compared to a reference tire which is rated 100) during the specified American Society for Testing and Materials traction tests on packed snow. The new standard helps ensure that drivers can easily identify tires that provide a higher level of snow traction.
A mountain / snowflake symbol branded on the tire's sidewall identifies tires that met the required performance in snow testing. The mountain / snowflake symbol was expected to be fully implemented on new tires by now. There are still some winter tires in the marketplace that meet the requirements to display the mountain / snowflake symbol but the molds used to make the tires were produced before the symbol was developed."