A good lofting program, such as Compufoil, can quickly and easily loft all the ribs, including ribs at specific locations. I don't believe the 3D wing modeling with slicing at the rib locations is any more accurate. What they are doing in essence, is making a digital version of Hunts "Lost Foam" system, without the cradle.
I've seen wings built in the cradle with ribs generated by a good lofting program. The lofted ribs fit was very accurate in the cradle.
A few other advantage of the lofting programs are, They are generated and inserted into the CAD program as DXF files. A few key clicks and less than 1/2 an hours time, can make them ready the laser cutter. There are functions that will set the spars size and location, type or style of leading edge, thickness of the trailing edge, web sizes, and lightening holes, size and location of jig rod holes, and more.
I can then render the entire wing 3D and check out all the fits, if I desire.
I can import another view, the top view as generated by the program, and use it to form the basis for the top view of the wing, again, saving time.
These features save a lot of time for me when lofting, and laying out a wing.
I've used the method Pat and Mark use, and it will indeed generate a good set of rib lofts. I've also used a profile of the airfoil curve at the root, and attached copies at the leading edge, then rotated to meet the trailing edge. ( think of building a digital I-Beamer) to generate a rib loft. The problem in my cases has been the amount of time it took, compared to a lofting program.
Add to the mix, the airfoil library, and the ability to generate, and modify airfoils, then use them to loft. I feel that the price paid is well worth the investement.