Some modern alloy wheels may be lighter, but the older alloy wheels/materials generally always weighed more than well made spoke wheels.
The spoke wheels also do have a little more compliance, whereas most alloy wheels are much more rigid. This helps with suspension/bump absorption. You can rebuild spoke wheels if you crack/bend a rim but one cannot easily do the same with an alloy wheel . If an alloy wheel is slightly bent they can be straightened with special, big, expensive equipment, but often they are expensive throw-aways. If you crash and bend a rim and snap a couple spokes, if the hub and bearings are still good one can easily lace up another rim and replace the broken spokes.
I've been waiting to see when we'll see some of the fancy carbon fiber mountain bike wheel designs get beefed up sufficiently to try out on motocross bikes - they'd still be pretty pricey though.
