I don't think the concern is as much about us all going to black and white, it's more about scenarios where the logo may not be able to appear in more than one color yet would still need to look acceptable.
While that does ring true when it comes to traditional branding, in our case I have a different primary reason;
The more edges, shapes, colors, shades, transparency layers, gradients and geometric vectors a logo has, the longer it takes for the human mind to process it.
Take street signs for example, imagine if there was an actual photo of people crossing the street rather than the traditional silhouette...