Ann,
Your years of experience as a B&W shooter is what "allows you to think" in grayscale, which is decidedly NOT the case for the vast majority of younger or beginning photographers, most of whom have been raised in an entirely full-color world...full-color newspaper photos, full-color magazine photos, all TV, all motion pictures, everything in full, glorious color. So, for an experienced B&W worker, like yourself, there are decades' worth of associations with B&W images--both making them, and viewing them.
However, for the newcomer, or the younger person, who has basically grown up in a world of full-color images everywhere (newspapers, TV, movies, internet,product packaging,etc.) there are a large number of variables that are virtually unknown to them. Shooting in RAW + JPEG, Monochrome Mode also preserves FULL control in the post-processing stage, since the RAW images will be captured in full R-G-B color at either 12- or 14-bit depth, depending on the camera; it is ONLY the JPEGs created in-camera will be irreversibly in B&W.
The huge benefit with shooting B&W mode in-camera for the beginner is that he or she can literally SEE a B&W image on a screen, right at the moment, and make the needed changes to exposure or composition, or camera positioning. Or to the type and position of the light. For example, Speedotron's Super Silver (metalized silvered) umbrellas make beautiful B&W portraits, because they create a significantly higher degree of specular reflections on human skin that either softboxes, or satin,or white umbrellas create....on the nose, cheeks,forehead,etc... when all the color is removed, those specular highlights add HUGE amounts of shape/contour/depth information in even a soft-lighted, B&W portrait. When the same umbrellas are used to shoot a full-color image, that same degree of specularity is often too much for subtle effects. The same goes for other light sources: 36x48 inch softboxes often create excessively FLAT, DULL, SHAPELESS lighting on images shot in color and then just simply converted to B&W. Truly successful B&W photography depends much,much more on the light's contrast, direction, and specularity/diffuse nature than color photography does.
20 years ago, I heard a TV interview with a veteran Hollywood director of photography. His basic premise in the interview was, "Well, he [the film's director] insisted that we had to shoot the picture in black and white, and our first day's rushes were so bad that I realized I didn't know the first thing about lighting for black and white. So, I called up an old friend who had many many black and white pictures, and met with him, and he told me a number of things I was unaware of." That TV interview has stuck in my mind for a long time. One of my "book mentors" was Gary Bernstein, Liz Taylor's exclusive portrait artist for many,many years. He always said that he liked to use slightly smaller light sources when the final image would be run in B&W, and to use higher lighting ratios most of the time. Good advice, I think. The Hollywood director mentioned that for his B&W film lighting setups, he felt that lighting ratios that were three to five times "steeper" than for color filming was about right for the look his director wanted to achieve.
Anyway---RAW + JPEG Monochrome is my preferred method with a d-slr. I get a B&W image, sharpened pretty heavily right off the card, which makes focus evaluation much easier once the images are downloaded to the computer, and gives me a good approximation of how the final image will look. Both Canon and Nikon images of 12-13 MP, scaled down to the MEDIUM-size, with sharpening set the High or Medium-High, give me a pretty good feel for what the images needs when reviewing on the back of the camera. Down-sizing the image from Large JPEG to Medium also give me about the size I actually WANT for many uses...if it's done right, the images look super.
The beginning shooter needs to see his images as monochrome/grayscale/B&W so he can learn how to light and compose for B&W. That's my point of view.