Iron Pony - I used a 20ga 870 for years and many people use all manner of field guns so it is not mandatory to use an O/U. Most skeet shooters are also field hunters, you will get away with using about anything and everything while playing the skeet game. If all I had was a pump gun (or semi-auto for that matter) and wanted to shoot skeet I sure as heck would not hesitate to use it. I have always found that skeet shooters are slightly different breed of cat than the trap shooter. In general trap shooters take their game much more seriously. They still are a fun group but slightly different.
Side-by-sides (S/S) are not commonly seen on the range (USA) but it is also not unheard of. I've heard several reasons for this. The relatively wide plain of two s/s barrels can make target aquisition difficult. However a good friend of mine insisted on using a S/S, Larry was a jumpy sort and felt that the wide barrels helped him to focus. He was a darned good shot so I never questioned him on his methods. Most s/s shotguns have small forearms and some barrel heat can be generated during a shoot. There is a barrel sleeve that I've seen used to shield barrel heat. Usually S/S don't eject cases they extract them. The shooter needs to pick the empty out of the chamber, where an ejector basicly delivers empties into the hand. But I am aware that in european driven bird shooting S/S are preferred. This is a sport where a handler loads the guns and hands it to the shooter, two guns, lots of in comming fast shooting. O/U have not prooven to be as handy for what ever reason I don't fully understand.
I shot for a while with a wealthy old gentleman who used old hammer S/S's exclusively. He had a beautiful set of guns, 12, 20, 28 and 410 all matched to their size. The wood was darned near black, the actions and barrels were silvery white. He had style. He'd show up in an English 3 piece suite complete with twill pork pie hat. Watching this old shotgun hand was a picture of grace. He was tall, not quite willowy, with a gracefull balance, he'd step into the box, action open , hammers cocked, load two rounds, close and mount the gun and call for the bird in one steady motion, singles were handled just the same as doubles. When two shots were fired he'd cock both hammers again, open the action and take out the empties. The next station was just the same. He looked, acted and moved like a wealthy english gentilman in every way, but had absolutely no accent.