You don't want a routine that's too processor intensive as the speed you would gain from culling would be taken up by the time it takes to calculate the culling.
This is the main point. As really, occlusion is only about saving time, speed.
Another problem I envisage is 'cataloguing' all the objects so that yu can refer to them to know which one's are to be occluded or not.
I am still very much in favour of portals, even though they are a little out-dated now (ah dont kill me for saying that!) But they are a good balance (as per quoted statement).