I always thought the point of him wearing a Peacekeeper uniform was supposed to be a nod at his assimilation. After all, before too long Aeryn grows to oppose everything the Peacekeepers stand for, too, but she still wears the uniform throughout the show. It's probably what you said above, wearing the uniform just makes his life easier as he moves through the galaxy, and the more people that mistake him for Peacekeeper the fewer people might find out he's not even Sebacean. Also, I know they showed some independent Sebacean colonies, but they seemed to be surprisingly few and far-between, which leads me to believe that a disturbingly large portion of that entire race is actually in the Peacekeepers in some form or other, which makes sense since that was what their race was created to do. Hmm, I don't know. I mean, on a meta level I'm pretty sure it's just that Ben Browder is incredibly attractive in them- I remember some interview where Claudia Black said she liked to drop things on set all the time so she could watch him pick them up, she said it was one of the main perks of her job- but I always thought that the in-text reason was basically that he was assimilating into things he didn't approve of, and that was just another part of his fucked-up life. Oh! and he also used Peacekeeper weapons almost exclusively, which would make using a Peacekeeper uniform easier since they were designed for certain weapon harnesses.