Digital BW, The Print

Does anyone know why we don't have a setting in our digital camera's that
would make the senor measure only luminosity? Am I wrong to think that for
B&W imaging that would be a very smooth, noise free image? Couldn't that be
a program feature? BTW I don't mean the feature in some cameras that
converts the images to B&W as an effect, but to genuinely turn off the
color. 

Wendel