Roxas only ever called on his Samurai once, and did Axel ever use his Assassins (I get the feeling that he did, but I can't remember off the top of my head)?
They appear right after DiZ freezes him in the last day of the simulation, clearly part of his attack force (which otherwise only consists of generics). Furthermore, after Axel is chased away from the simulation, you don't see Assassins again for most of the game. They only return after Axel dies, if I'm not mistaken, and certainly not in force, and have presumably been missing because Axel's running around trying to find Kairi and the Org perhaps didn't regain control of them until he died.
I think if helper Nobodies had been included in Days, there could have been elegant explanation for why there aren't helper Nobodies for the Castle Oblivion crew: they might have been introduced to the game after the events at CO take place. Only the living members get helpers, problem solved, Xion doesn't get her own (she'd probably get Samurais to cement her being a gameplay clone of Roxas), adding further hints that she's not a "proper" member. Alternately, Days could have added new helpers but we know that didn't happen.
Off topic, but frankly, the lack of helper Nobodies in Days always annoyed me (and portraying them as "practice" enemies didn't make them seem like allies, either!). I get that the mechanics just didn't work, but when the Nobodies show up in 2, they clearly know Roxas, call him "Lord," and the Samurais respond to him even after his "death." Couldn't they have at least showed up during Days' conversation sections, hanging around, being creepy? (In the manga, Dusks doodle on Roxas' face when he's unconscious, I think that's fantastic)