I think pretty much any modern digital camera will work just fine for B&W work. Color RGB sensors have gotten very, very good.
That said, as the owner of two Leica Monochrom's (the original CCD version as well as the newer CMOS model), I can affirm that, for me, there are both technical and creative benefits to using such a dedicated camera. It prompts you to see in tones of gray, much like when you load a roll of Tri-X in a film camera. Actually, the whole process of shooting a dedicated-monochrome camera is very much like shooting film. And, yeah, I was a naysayer when Leica first announced the Monochrom back in May of 2012. I thought it was indeed a contrived, 'statement' camera, with little practical use. I would have laughed at the notion that three months later I would not only own one, but that it would quickly become the finest, most addictive, most creatively inspiring camera I had ever owned.
Alas, I agree with the sentiment that the market for such niche cameras is exceedingly small. I wouldn't count on seeing more of such models, from other makers, anytime soon.