The advantage of portal rendering is the simplicity to code it and the increased speed in your game. For each portal you make a clipping frustum out of the wall polygon, then render the sector that the portal connects to. The initial frustum is the viewing frustum used by the camera. That type of clipping is 3d, the 2d counterpart is where you calculate the 2d rectangle of the portal on the screen and do 2d clipping.