Seems to me that paper and long steel mandate a slug, clays and steel are birdshot targets. Distant spinners and such should be shot with whatever heavy load thats allowed, be it Prairie Storm, buckshot, turkey loads. I don't get requiring buckshot in the WSB though.
I've only been to one match requiring buckshot, and it wasn't because it was needed or a clever stage where buckshot would give you an advantage. It was just a stage designer that ran out of creativity and substituted buckshot just because he could. Half the shooters didn't have any, and the other half, like myself, have been carrying the same box around for the last 4 years wondering why its still in the range bag.
Maybe I just don't get the buckshot thing because I'm relatively new to the sport.