Sorry for my slow response. There is a lot to digest in these terrific responses, and a many more factors to consider in the history than I had anticipated. For instance, I had not really considered the EMD NW3/5 models as precursors to the road switchers, or as (attempted) competition with the RS-1. But now I notice that Louis Marre does say, in his Diesel Locomotives: The first 50 Years, that "The NW3 and NW5 were marketed as passenger terminal switchers, in competition with Alco's boiler-equipped RS-1" (p. 25).
Aside: There is an NW3 on display at one of the stops of The Empire Builder on the Great Northern route--I have a photograph of it. There were only seven made, all for GN, and only 13 NW5s, the latter after the war; not much competition for the RS-1, which was going great guns by that time.
Then there were the various center-cab terminal units, and GEs center-cab xx-ton switchers (even off-center cabs which look like later road switchers: see Marre, p. 188), which leads one to create diagrams like biological-evolution clades, with offshoots and dead-ends here and there.
Then there are the factors that went into road-worthiness, i.e. road trucks, steam boilers (for passenger operations), union rules, toilets, and railroad demand. Still, when you think about it, it's interesting that the design of road diesel-electrics initially leapfrogged the road-switcher evolution altogether. EMD's cab units were, I would venture say, just streamlined (after the art-nouveau fashion of the '30s--viz. the Air-Flow autos from Chrysler) versions of the utilitarian box-cab locomotives, but quickly took over directly from road steam (interrupted by WWII). As pointed out above, they were really useless for switching and even branch-line service (where switching was required), with B units even more limited. It looks like it was really the RS-1 that bridged the gap between diesels for yard work and road work.
I'm tempted to wonder whether, if the railroads had stuck to box-cab diesels, with a cab at each end--or for that matter, center-cab units--whether those would have evolved directly into the road-switcher. But the streamlined diesels were somehow more appealing, especially the sharp-nosed EMD E units, which the advertising people loved.
I want to delve more into reading the history, but I greatly appreciate the discussion so far. I will only note that with the wide cab development that RME discusses, we have come full circle. Despite their front-and-rear platforms, those behemoths are not really at home in the yards or branch lines, are they?