For this generation 4x 8pin might not be a big deal, but in the future you could be looking at 6x 8pin or more. Also there are additional benefits when the 12+4pin is paired with an ATX 3.0 PSU.

You have to remember that with an 8 pin connector it's really just 6 pins for the power delivery.

They could have done something like they did with the dual 8 pin cables, basically made a pigtail 12 pin with each connector at the GPU just doing 300w for 600w total, but you'd still have 600w from the single 12 pin at the PSU, plus they were trying to reduce the number/size of connections on the GPU.