The optical design also has a lot to do with the quality of the oof areas, not just the aperture blades, although they can help a lot. I personally like using shallow dof for certain photographs, gives a more pleasing 3D effect, so the quality of the oof areas are very important (notice that I haven't used the "B" word yet)
I suspect like others have stated, that the dollars have caused the modern trend towards minimal blades, but also the auto diaphragm function must require less moving parts. It is pleasing to see that some manufacturers are coming back to designing lenses that have a more pictorial quality rather than clinical sharpness (Cosina/Voigtlander are a good example)
From an aesthetic point of view, those old multi bladed apertures are a thing of beauty.