Lets see, you brought up warriors, rogues, and dk's as examples of classes having similar specs that needed diversity just as much if not more than hunter.
Warrior- has 3 specs, one of which is WILDLY different than the others. Can fulfill two different roles.
Arms and Fury, Different how?
DK- has 3 specs, one of which is WILDLY different than the others. Another has a pet as well. Can fulfull two different roles.
Frost and UH, Different how?
Rogue- Specs play differently as well. Sub plays NOTHING like mut or combat and vise versa.
Different how?
Again, you're making a terrible argument for an idea so off the wall like "they couldn't come up with 3 bows so they chose to make an entirely different spec as it was way less work to do the former than the latter."