I'll add a bit of my own perspective on this discussion. I work with two current monochrome cameras: the Phase one Achromatic+ IQ3 Medium format digital back, and the Leica Monochrom.
Leica Monochrom - Leica Summilux-M 21mm f/1.4 ASPH ISO 320 10s f/16
B&W has always had its adherents, and monochrome digital sensors have a number of distinct advantages over Bayer Colour filter array equipped sensors. Right off the bat: Images are significantly sharper than any bayer camera can produce due to there being absolutely no interpolation or de-mosaicing needed to be performed. Hyper-spectral imaging is more common - colour sensors require rather heavy handed UV/IR cut filters in order to be able to replicate colour as accurately as possible, without UV/IR contamination. In B&W if one chooses, this concept can be safely thrown out the window.
The Phase one Achromatic IQ3 digital back has a very wide spectral sensitivity range - UV/IR cut filters are commonly a part of the sensor cover glass but with a request from Phase you can opt out and use the MFDB sans filter for hyper-spectral imaging which has applications in forensic and fine arts photography. Also extreme camera movements that cause lens shading to occur aren't as degrading to image quality as it is on bayer filter equipped sensors, lens shading appears as just another form of vignetting.
Leica Monochrom - Leica Summilux-M 35mm f/1.4 [pre-asph] ISO 320 1/640 f/11
One of the drawbacks with digital monochrome sensors is the sensor has to be pretty much perfect, there can't be any glaring faults on it. If there is: the image won't be homogenous. Colour sensors in many ways harbor many defects which can be cleverly mapped out in hardware by using an adjacent pixel - you can't get away with this to the same degree on a monochrome sensor because each pixel element is used in creating the image, such mapping would be just shifting the stuck pixels around.
By removing the light attenuating Bayer Colour filter array has the practical upshot of increasing the sensitivity of the sensor. In the case of the Leica Monochrom: the sensor used is essentially identical to that of the Leica M9 - however, the base ISO of the M9 is 160 - the Monochrom gains a full stop advantage - ISO320 as its base ISO.
The rather high base ISO brings with it a number of advantages, and disadvantages : really fast lenses such as the Leica Noctilux are not really needed as the image quality from the monochrom is equal to the M9 at its base ISO - you can get away with using slower glass in dimmer light. The relatively low maximum shutter speed of 1/4000th means using lenses with apertures wider than f/2 in daylight is not possible without using an ND filter. But also in daylight there is zero chance you will have issues with maintaining high shutter speeds. The High base ISO of the monochrom makes creating long exposure images difficult. The sensor gains a full stop noise advantage over the M9, ISO 10,000 on the Leica monochrom is
superb . The luminance noise from the sensor is tightly packed, it looks a lot like 120 format Ilford HP5 pushed to ISO3200 in Rodinal.
Pentax K5IIs - SMCP-K 50mm f/1.2 @ f/1.2 ISO 80 1/250th [ B&W conversion done in post]
The Leica monochrom being a rangefinder means that you can put colour contrast filters on the lens without it disrupting your perception - but you have no post editing options to change this like you do with colour bayer equipped sensors with separate colour channels.
Monochrome sensors are a niche market - but it is a
diehard market*. A market that, as far as I can see: isn't being served by any of the 4 major manufacturers. Pentax should exploit this and perhaps create a K-02 with a full format monochrome sensor, or a version of the 645Z with a Monochrome sensor.
Originally posted by Adam monochrome sensors have issues with highlights that color sensors do not
Would you care to expand upon that Adam? From my experience the highlight retention capacity of the Leica Monochrom Vs the Bayer equipped M9 is pretty much
identical. Both cameras are based upon CCD sensors which, truth be told aren't anything to write home about when it comes to dynamic range.
* if you need any proof: check out any dedicated digital B&W photography forum.