.
It doesn't really matter what power he uses, especially since as far as we can see, Roxas is about the only person that uses Light in the same manner everyone else uses Darkness.
The only other person I can think of would be Minnie, but that's explained by Roxas being from simply Sora's Nobody, to being related to Ven who might be a BWHoPL (...wow. I never really thought about it like that ).
In other words - Nobodies are all related to Darkness; Roxas using Light as a weapon makes him not that much of an exception. He's made by Darkness.
If Roxas is Sora's nobody they should have similar abilities but, why is it that Roxas can control Light and Sora cannot.