Ted,
You probably have answered your own question. I agree most double fw holes have a narrower fw for the shortcut option and wider for the longer option. As to how to guard the green, I think traditionally, the shorter route got the open front, but not always, i.e., Riv no. 10. And, I have heard Pete Dye say he preferred to guard the shorter route twice to negate length on such holes.
Taking out specific examples, there is nothing that says when a hole has two fairly distinct routes off the tee, that one needs to be clearly dictated by the gca as greatly preferred. Unless the challenge of a greatly preferred route is nearly 50-50% to pull it off repeatedly or on individual days in specific wind conditions, and is very tempting, then you often end up building a one dimensional hole, that happens to have an extra fw to mow on one side or the other.
So, at least some of the time, offering two routes that provide equal but different results and challenges off the tee could be considered more strategic, with golfers choosing the best route more based on their game, and less on the course itself.