Double-butted spokes have been around for a very long time. I remember seeing a bike from around 1910 with straight-pull double-butted spokes. The two 1930s Reyhand featured in the latest issue of Bicycle Quarterly have double-butted spokes, too.
The advantage of thinner spokes is aerodynamic on the one hand, but also making a stronger wheel by allowing more spoke elongation for the same spoke tension. As a result, the spokes are less likely to go slack as the wheel hits bumps. This means they work less and fatigue less. Double-butted spokes put extra material where it is useful - on the threads and at the elbow. The latter point is where spokes usually break.
A minor disadvantage of thinner spokes is that they wind up more when you true the wheel, so they require more care and experience. -- Jan Heine Editor Bicycle Quarterly 140 Lakeside Ave #C Seattle WA 98122 http://www.bikequarterly.com