There was enough complaining about scene skipping and the like this season alone. Even if the Kingsmoot was moved to S4 (Which I do think would be a good idea), condensing AFFC/ADWD into one season would leave no room for anything to breathe. The whole thing would be rushed to hell and back and it would be a complete clusterfuck. There's just way too many characters/plotlines to fit into one season alone without butchering them. Arya/Jon were already a victim of this in S2, and trying to fit everything that goes on in the North/South/East into one season would just make matters even worse. Plus I wouldn't be surprised to see non POV characters here and there as well (Like Tywin and Robb this season). That, and I would guess we'd see at least one of the two battles from TWOW as a finale. Also, the budget's stretched enough already, there's no way it would allow to have everything from AFFC/ADWD in one season considering the amount of CG they would need for some things like the dragons.