My O/U trap, skeet, and sporting clays guns have the lower barrel ported. Since we normally shoot the lower barrel first, it does keep the muzzle from rising...slightly. It's also louder, dirtier and will shave off some of the wad creating plastic build up.
I also have several single barrelled guns that have ported barrels. I don't see the advantage in them that I see in the O/U guns. It doesn't hurt anything but I don't see that it helps either.
I do have one exception to what I've seen in other barrels. I have a Benelli SBE barrel that I shot from the factory and then I had it ported. After I had the barrel ported, the pattern was more open,using the same choke/load but a bit more even than before. It's one of my better shooting Benelli barrels but it is louder.
Lastly, my Benelli Vinci with a non-ported barrel has less muzzle jump than any shotgun I've shot. I think it has more to do with the overall design, maybe the balance point, than with the ports.
Thanks,
Clark