A monocromatic sensor does have better resolving power than a regular color one, but that is the only positive thing about this "new invention". What is lost is the infinite manipulation possibilities what PS, Lightroom etc give us to adjust how the colors are interpreted in the B&W image. This is a huge downside, I mean HUGE. The person buying this camera must fulfill several criteria: wealthy, likes B&W, interested in photography, does no know anything about digital photography and the B&W manipulation possibilities it affords. How many of those are there in the world?
Not if the colour sensor is an 18MP Foveon sensor - then they'll have the same resolution, only that one has colour and the other doesn't.
This is a very, very specialised tool, though, and may not even suit most people who primarily produce monochrome output. With a regular colour sensor, you can apply colour filters in post-processing, including using different filters in different parts of the image. With a monochrome sensor, you need to use colour filters at the time of shooting (there goes your ISO and resolution advantage already), with no option to fine-tune it in post-processing.
But, just like Canon's recent 60D astrophotography camera, I don't think it's meant to be a high-volume product (even by Leica's standards). It's certainly not the replacement for the M9.