With Smith & Wesson, the cost savings is probably the driving force. Quite simply, one barrel can be fitted to a wide variety of barrel shrouds and this can reduce production costs.
However, for Dan Wesson revolvers the driving force for the suspended barrel was that it produces a more accurate revolver. Gun barrels do move in response to a bullet pasing through them. When the barrel is retained at just one end, that movement is basically a whipping motion.
Bull, or large diameter, barrels are an effort to reduce this tendancy to "whip". So are top ribs and full length underlugs. However, even with these measures a barrel mounted at just one point will "hinge" where it is mounted to the frame because of the reduction in dameter at the mounting point.
Now, with a suspended barrel you have a different result when the bullet is fired. Because it's mounted in a larger diameter tube with a wide bearing surface on the frame, the the tendancy for the muzzle to "whip" is reduced. In addition, a barrel that is under tension has less tendancy to exhibit mid barrel movement than a barrel that is not under tension. Basically, it's a package that is stiffer than other methods of mounting. Because of this, it's possible to produce a revolver with accuracy that matches or exceeds that of a very heavy bull barrel without the extreme weight gain of a bull barrel.
Which is one reason why I chose to purchase the 620 for a 357 over the 686 Plus. In terms of system dynamics, it's a superior system of mounting a barrel on a handgun. The other reason is that I don't like the way a full length underlug looks, however that didn't stop me from purchasing my 610.
Now, off a hard bench rest my 620 is the most accurate handgun I own. On two seperate occasions, shooting from a hard bench rest in single action, I have managed to put 3 rounds into a group that could be covered with a dime at 40 feet. The best I've been able to do with my 610 at the same distance is a half dollar group. However, I've been sorting out a leading issue with the 610 when used with the 40 S&W so I may improve on that down the road.
Point is, there are some distinct advantages to mounting a gun barrel in this way. However, I have yet to shoot any revolver in double action anywhere close to it's potential for accuracy, so for most of us it's a moot point. Still, it is sometimes fun to play with a really accurate handgun in single action. You should give you model 64 a tryout at this before you decide it's an inferior gun, the potential for accuracy will probably surprize you. BTW, my 620 groups really well with Federal 130 grain FMJ 38 spl.