I'm not sure how it's questionable, you can definitely put much more torque on the pedals when you can push down with one foot and pull on the other, for sprint

I'm not sure how it's questionable, you can definitely put much more torque on the pedals when you can push down with one foot and pull on the other, for sprinting in a big gear or climbing. The questionable part is, do people apply pressure on anything but the down pedal stroke when they're seated, and riding normally. At least for me, the answer is no. But they're still nice to have, especially on a single speed or fixed gear. And for fixed gear, flat pedals are unsafe. They give you a lot more control.