The choice of color space can have a large impact on noise (which makes most of the “objective tests” of digital camera noise misleading at best to anyone looking for real image quality).
More noise, but possibly posterization
At best, the effect of using a narrow-gamut color space such as sRGB or AdobeRGB simply means more noise in one color channel. But in some cases it means posterization of image detail, with an accompanying color shift and destruction of detail.
Black and white conversion
More noise in one channel might not matter for an RGB image very much, but it matters a great deal if one wants to extract a black and white image from a particular channel, or mix channels together (a noisy or posterized channel will pollute the result). For black and white conversion, the individual channels offer a useful hunting ground, but sRGB is generally so awful that it often is best avoided.
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