i agree completely... i only use the 3 1/2" for turkeys... and thats the only 3 1/2" 12ga I own... i like the shorter action on 3" bps or even my 2 3/4" model 12... you can shoot them faster, cheaper, and they don't hang up or get short-shucked... 3" is all the power you need for waterfowling, and if you're not going to shoot them, don't buy a 3 1/2" gun...
i would also say that my personal opinion from my time in public hunting areas has led me to believe that the 3 1/2" guys are usually the guys who can't call, don't know how to work ducks, or are too lazy to carry decoys... they have to resort to sky busting birds working someone else, thus they need the extra power and range...