Rocker facilitates cornering, straight facilitates going straight. The progressive edging that is popping is just easier to get done quick with a defined stiff curve between your feet. No energy is first spent on bending the board to initiate the pop.
Your right its less efficient for planning but that's the trade off. Efficient boards I find less fun. I don't have kite bigger than a 10 and my only twintip has more rocker than any production board on the market. It's stiff and works best with boots cause its 139x43. That's enough size to make up for some efficiency and I won't even take it out of the car unless its a solid 15 knots. If I had the patients for a 12m I'm sure it would be fun down to say 12 or 13 knots.
Like the donkey dick a lot of gear is designed for the beginner. Board are made to sell, and what sells is upwind at the expense of almost all else.
Pros compete on flat boards to max speed and get back upwind quick for timed heats. Hadlow immediately went to a far more rockered board once out of comps and into boots. Same with all the non comp guys like Dre, Sam Light, Mo, Predator, Mauricio, etc. these guys can all pop huge on almost any board, but choose to ride rocker.