i have seen a couple broken cataracts but it's rare. the main idea behind having a spare in the boat is if you jam your oar into/behind a rock in fast moving water, it can catch, lever upward, spread your oar lock and pop out of the boat.

having a spare at the ready allows you get through the rest of the rapid in one piece and chase down the oar you lost.

in any case i still think 9'ers are too long for a raft, but that's just my opinion.