SW's policy seems fair but I don't see any reason why 2 POS couldn't just buy one extra seat. POS get to preboard and under the present policy they will get 2 seats together, getting 3 seats together isn't that much different.
Good point. Here's another (ir)relevant question. I've been on other airlines' flights, where due to weight restrictions they had to push some people off and take off with empty seats.
How close are the margins on SW aircraft? Would a troupe of COS heading to the Weight Watchers convention blow the plane weight, since only cargo is actually weighed (and passengers just estimated) when calculating takeoff weight?