O/U isn't essential to shoot anything - including Skeet .....
O/U's are common on registered Skeet fields - because the guns get a lot of use ( 8 - 10 boxes a week are common / or 10,000 shells a year )...and a good O/U will last for years with that kind of usage.
But that isn't to say you shouldn't shoot Skeet with a pump gun or a semi-auto if that is what you like / and the gun fits you.
Personally, my all around gun for Sporting Clays, Skeet and bird hunting is a Browning Citori O/U, ported barrels, 30" barrels, with a parallel comb and an adj comb insert. I like an all around gun to be around 8 1/2 lbs - so it swings smoothly - well balanced - and for me that is a Browning Citori XS Skeet model. I happen to have that 4 of these guns - one in a 12, 20, 28ga and a .410 / while 4 guns may not be optimal - it works for me.
But my backup Skeet gun, traveling bird gun, bad weather gun - is a Benelli Super Sport 12ga, 30" barrel, semi-auto. It does everything pretty well.
For Trap, I go to a longer and heavier gun - a Browning Citori XT O/U with 32" barrels, ported and around 10 lbs ( with a parallel, adj comb ). Trap has less left to right barrel movement - so a heavier gun is a plus. Its not that, the XT isn't ok for sporting, etc - but with 32" barrels and 10 lbs - its a little like trying to swing a big ole sewer pipe at hard crossing pairs like on station 4 in Skeet.... but it can still be done.
O/U's are reliable - if you can shove a shell in the chamber and close the gun - it will probably fire. You can have 2 different chokes. If you reload - your hulls don't hit the ground ( and get dirty and wet )..., triggers are good on the better guns, there are more stock options ( like adj parallel combs ) ......