i I thought it interesting that I have the image option of monochrome in my Rebel Xs but NOT in the most recent purchase of a 1DS MkII, with seemingly eons more technology built in. Not a complaint or buyers remorse, just an observation. And yes, I "assumed" it would.
I think the primary reason would be the customers being marketed to. Many of people who buy the Rebels will shoot jpg and the monochrome option is handy. Those shooting with a 1d or 5d would likely be shooting raw which is going to have all the color information in it anyway, there is little reason to apply any processing like that in camera when the consumer is likely going to want to control that in lightroom or photoshop, after dropping a few grand on a pro body. My 2cents anyway...
My guess is the same as Johnsey.. Rebels are entry-level. Most people buying more advanced cameras should be shooting raw, and not limiting themselves to a crappy jpg-only 'print'. Of course, obvious exceptions being sports/news photographers, for instance. Out of curiosity though, does a monochrome option show up when selecting its RAW+JPG mode? I agree it does seem strange that there would be no such option. Is there perhaps an option for monochrome in any possible in-camera raw conversion 'utility'?
You might consider trying Nik Silver Effects Pro, it's free now... it is unlikely that it will be updated though... https://www.google.com/nikcollection/products/silver-efex-pro/ Best, Steve
Since the Monochrome setting is for jpg files anyway the better option is to always shoot in RAW to give you a lot more control over your post-processing, including tools mentioned above for much better B&W options.