Very helpful replies from both Andy & Glenn.
Converting some of my photos to black&white (monotone might be more correct) is something I’ve considered doing for some time now but have yet to find the time. The current experience came about as a result of digitizing (i.e., scanning) a very old and degraded monotone photo received from a friend. Without knowing any of the above thoughts it seemed fairly natural to choose the “grayscale” option for the scanner output. However, purely from a curiosity point of view I also produced an rgb version with the same scanner. As it turned out the rgb version came out as a very faithful representation of the degraded color of the photo whereas the grayscale version simply looked better (to me).
As it turns out browsers are NOT the only software to have difficulty with grayscale. Because of some difficulties encountered printing the result I wanted to use the more sophisticated software associated with my photo printer. However it is supplied as a plugin for software used to develop raw image files. That software was worse than RT in the sense that it couldn’t even open a grayscale file. I’ve had similar problems in the past with the same software failing to recognize multi-layer .tiff files. I used to solve that problem by using RT to read (import) the multi-layer files and write (export) a file that could be processed by the software used to invoke this printer plugin. As it happens RT was also similarly successful at solving this problem by producing a file (in rgb format) that could be processed with the plugin. I just happened to notice that it did this by converting to rgb and being ignorant of the above considerations thought it would be better to retain the grayscale format produced by my scanner if only because of the smaller size.
The above pretty well explains why I should want rgb format when it comes to working on my photos and I now pretty well understand why this software (e.g., RT and others) that is primarily intended for developing photographs has NO need to produce grayscale files.
Thanks for the enlightenment.