What it really comes down to is physics and not debate. Flow rate (Q) is velocity (V) times the area (A) plus friction factors but Q=VA will work just fine for this discussion. The fact of the matter is there is a reason that they put dual runner intakes on engines and it isn't because they look good at least as far as the factory is concerned.
If they could get the job done as well with a cheaper to manufacture single runner intake they would. Here's a neat little site I found that talks about some of this stuff:
http://www.eng.fsu.edu/~shih/eml4421/students'%20web%20pages/manifold%20tuning/intake.htm
There is a balance in a conventional intake manifold and carb set-up between the turbulance needed to keep good fuel delivery and the boundary layer formation. You get the walls of the runner too smooth and the fuel condenses on the wall, too rough or too small and the losses become too high to get the flow rate needed too large and the velocity is to low to properly suspend the fuel and fill the cylinder. Thus the multirunner intakes on a direct port injection system. No fuel delivery issues to deal with in the runners, the best of both worlds in velocity over the rpm range.
Great book to pick up used on Amazon or one of those: Cams, Valves and Exhaust Systems - Peterson 1968