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In fact, carbon fibre is a dumb material choice for spokes however you use it. Steel is better, simply because the spoke a pretty much a pure tension element, and steel has a high Young's modulus, allowing adequately stiff wheels to be built with a smaller total cross sectional area of spokage than other common engineering materials. Since the spokes contribute hugely to the aerodynamic drag of a wheel, getting the spoke cross section minimised is half the battle in making a fast wheel. The trivial weight penalty of using steel compared with lower density alternatives is massively outweighed by the aerodynamic gains from thin spokes.

Total cross-sectional area is for the most part irrelevant if they are aerodynamic spokes though ( but I'd agree with you completely if they were cylindrical). This only really affects skin friction, and the reduction in form drag by making them aerofoils far outweighs this small increase in skin friction. Cross winds would suck though.