Ooooh, definitely agree there. He really wouldn't find much use for them because they're Light-based while MX is more into Darkness, and I doubt he'll bother finding a pure-darkness being to fight the Princesses without said being being driven away because of all that light, not to mention it's a Disney game and none would dare harm a girl, let alone the Princesses.
Ok, let's see what we know of the 3 said individuals...er...beings...or half-beings, whatever:
Xion - Roxas/Sora imperfect replica, manifestation of Sora's memories, looks like Kairi because Kairi is the Most Important Person to Sora. Not a Nobody, just a bunch of memories turned human. A flawed sample at that.
Roxas - Sora's Nobody. An anomaly to being even a 'Nobody' with the existence of a fragmented heart within him. Capable of feeling emotions because of said fragment. Has "2 1/2" composition of 'being-ness.' The fact that he existed alongside Sora is unusual, more so with Ven. (Might he have 'faded' within to when he joined Sora the first time? Was never shown on scene to be in 'close proximity.')
Namine - Kairi's Nobody. An anomaly to being even a 'Nobody' because of her unusual powers, birth, looks, composition, etc. Capable of feeling emotion, but has no basis. Unknown composition of 'being-ness'. Vexen has a folder that contains studies on Namine. The fact that said folder even exist, and coming from an academic, infers that Namine is indeed an unusual entity that requires more in-depth analysis. Fades when in close proximity to Kairi.
On another note: As for the activity of a Nobody when reunited with their True Self:
They might have some form of volition with their own will, there are many scenes in the game that prove this. They can probably act independently, albeit at a very limited scope.