But you do skip content with dual spec. You skip a grind. Tbc is about grinds. Removing a grind makes the content consumption happen even faster. Not to mention the opening of dual spec let’s raids optimize even more than they already do to increase content consumption speed even faster.
Tbc isn’t a game designed to switch roles whoever you want, it’s designed planning when you switch roles to maximize the use of the switched role. The game designers of the time even said no to spec swapping (which is what dual spec is) because they didn’t want people switching 10 times a day whenever they wanted with no real cost or downside. Dual spec goes against the design goals of the devs that made tbc. And any change made should NEVER go directly against the design of the game.