O/U's are my primary gun for Skeet, Trap, Sporting Clays and for bird hunting .... Quail ...Pheasant ...and even waterfowl ...
I use a different gun for "Trap" ...and while I have break open single barrel trap guns, like the Browning BT-99 or the older BT-100, I prefer the weight and balance of an O/U for Trap - like the Browning Citori XT Trap with 32" barrels.
I like my O/U's in the field / hunting birds ...mostly because I shoot them all the time - like the weight, balance, feel on follow thru - and I use all 30" barrels on my field hunting ....
I have pump guns and semi-autos as well ....but I'm often as sucessful with 2 shells as I am with 3 .... Maybe because I know I only have 2 shells ...I don't slap that first round out there ...without making sure my feet are set, my mount is ok ....and I can execute the shot ....so I don't see the O/U as having much of a downside in the field.
So no, they are not only for Trap ....( in my opinion ) .../ although I do like the longer and heavier gun for Trap (32", around 10lbs ) vs everything else ...
30" barrels and around 7 1/2 - 8 1/2 lbs /because Trap has less barrel movement left to right vs real hunting, etc ....