Another advantage to scanning B&W negatives in color can be sharpness and resolution.
Each of the 3 channels in a scanner has a different sensitivity (kind of like having a different ISO in a digital camera), so each channel will have a different amount of noise. And when scanning negatives, the noise will show up in highlights, where it's more noticeable.
Also, one of the channels may be sharper than the others. This is often the green channel, since it is in the middle of the optical range, and less expensive scanners may have lenses that are better corrected here than at the extremes of red and blue. Typically, if you scan as B&W, the scanning software may take some sort of average of the 3 channels, which may not be optimal.
Software like Vuescan lets you inspect the 3 color channels individually, and choose which one you want to use for B&W conversion.
--Greg