I used the feature all the time.
I set the camera for Monochrome and shoot RAW. This gives me a monochrome embedded preview. The "Q" key allowed me to toggle to a locally rendered color preview.
I could live without this feature if there was some way of quickly toggling between the camera rendered preview, and a default rendering of the RAW file.
I don't use DNG files, and don't feel a desire to install Adobe software on my computer. If I liked Adobe software I would be using Bridge or Lightroom, not Photo Mechanic.