Ben,
The determining point is whether it is an equitable nine-hole fee..a local course charges $25 for 9, $35 for 18. That give me no desire to play nine.
Matt,
Did the guys at Torrey Pines indicate that the percentage included league play or not? At many courses in the USA, I would guess that after-work league play during the week accounts for a much higher %%% of all rounds, and this play is completely 9-hole. However, some courses discount the "other" nine for ladies and guys who want to arrive early to play 18 total holes; this also might affect the numbers.
The USA is the land of influence and numbers. We have [certain] numbers fixed in our minds (Roger, what perfect symmetry exists with the number 18?), which is why we prefer medal to match play, and we are influenced by tournament golf, which sets 18 holes as a number to play.
For many, golf is seen not as a holistic part of the days (work, golf, home) but rather a mini-vacation from the grind (honey, I'm golfing today.) If it were the former, 6, 9, 12 hole go-rounds might become more common. Since it is usually a portion-of-the-day-reserved affair, mainly on the weekends, 18 holes becomes the norm.