If R2-D2 is a key part of the Star Wars universe, then that goes just as much for C-3PO, and if the astromech is going to be around, then it follows Threepio will be as well. It's a similar story, really, in that he'd be under Organa ownership, and be able to play a part in the Rebel Alliance's plans, where he'll be valuable for his linguistic expertise. He wouldn't be needed in the actual heat of the battle, but could have a crucial part to play back in the command room. Most of all, though, even if it's a fleeting appearance, it really wouldn't feel like a Star Wars film without him.