Would a perfectionist shooting in black & white for artistic value find a need for a special camera? I'm thinking of the Leica M monochrom or the Sony a7r.
The responses have cleared my mind from shooting in B&W to converting to B&W. This changes the question to what full frame camera will do the job best? Under $3k with lens.
The Leica is a huge mistake. Don't even know what Leica was thinking. Very dumb on Leica's part! You can learn why on my blog were I just happened to write about it recently:
Controlling the translation of color to gray tones of a photograph after the fact of exposure has always been a desirable goal. If one could somehow apply that colored filter in the darkroom, instead of the camera, mistakes could be corrected and different options tried. This possibility did in fact exist with film. In theory… From my blog
Capturing the image in color allows the photographer to apply the gray conversion she wants. And this is best done at home, with Photoshop and maybe even masks. None of this could be done if the image was taken in B&W in camera, as then the gray conversion is fixed.
What you need for serious B&W work is the biggest sensor, most megapixels you can afford. Minimum: full frame 35mm sensor, 25 megapixels. I am currently using a Sony Alpha 900. It is adequate, but just barely. When I have money I am going after the Nikon 810. I would have stayed with Sony, but they decided to abandon me, leaving no upgrade path. You might want to think about that before buying the Sony. Sooner or later you will have to replace everything. Features don't matter. Pretty much all cameras at that level have the same features, in one way or another. Buy the very best lens(es) you can afford.
The advantage of a B&W camera is that there are no color filters, therefore the sensors catch more light and no demosaicing has to be done. This leads to far better image quality; the more light the sensor catches, the less noise you have and demosaicing algorithms can obviously only approximately render the RGB values of a pixel (and you get worse results the more noise there is).
The advantage of ordinary cameras is that you have the freedom to define the mapping from a color image to a B&W image, as pointed out in the other answers. If you are a perfectionist using such a camera, then you should consider using dcraw to extract the raw non-demosaiced files. Since you are going to map to B&W anyway, you can do the demosaicing more efficiently for that purpose.
Out of the box, a specialist camera like this doesn't make any sense.
In theory Moire-effect might go away, or you might capture more data on tonality, so the sensitivity could be higher. But none of these are actual problems in actual photography anyway. (They can be fixed in post or by buying a flagship camera with high iso-ratings at low noise.)
Many people would consider this to be mere marketing on Leica's part.
I agree but think there is more to it: Simplicity. By buying this camera you are deliberately limiting yourself to taking black and white photos. Back-to-the-basics style you can take only these images and it becomes a very conscious choice.
Compare it to the Painterly-style in painting, or Abstract art. These were artists who could paint "properly" but instead they chose to focus on something specific (colors in the Painterly case).
The Leica MM has a different sensor, which allows a far better B&W image to be made.
To ask for a "perfectionist's" camera with lens for under $3K with lens is completely unrealistic. Perfection doesn't kick in until you get to the $20K range, at least, plus lenses. The Leica S system is a place to start looking, if indeed you are a 'perfectionist'. $3K will get you a decent used Leica lens, if you are lucky.
