I have a suspicion that Fai won't use his powers because of something that happened in his country before he left. I think that's when he decided never to use his magic again (after sending himself to Yuuko).
Or maybe he is just afraid his powers would be out of control. I think it's more than that though.
Maybe both things. Remember the bodies outside (or was it inside? ...) the castle in Celes (manga), I´m not sure if there´s a war or something in there but I get the idea Fai was involed in the country´s happenings, he was court wizard after all, and even when I never get the idea he would like harming people, maybe he was forced/deceived into it.
About magic I still remember at Koriyo, when the group asked for Yuuko´s help to enter the castle which was protected by a spell, Yuuko´s first response was "you don´t have to ask me, Fai can use magic", he starts saying he gave her his tattoo but Yuuko interupts stating that the tattoo was just a source of magic to stop other magic, not the main source of his powers, but he still replies he has decided to stop using magic without it. (I´m taking this from scanlations, but I hope it´s not far from Del Rey´s version) That´s why I´m almost sure he keeps on trying not to use magic because otherwise something bad might hapen. Still I think eventually we will see him using it, either by loosing controll or developing stronger bonds towards his companions and trying to save them.
Also I´m surprised no one has ever asked him how he knew about the existance of Yuuko and if he knows more about her. We know Yukito and Tomoyo did know where to send Shaoran, Sakura and Kurogane in order to go around different worlds. When they arrived to the witch´s place they didn´t know who the woman was, but Fai on the other hand is the only one who transported himself by his own means (that´s the last time we see him use magic in fact) meaning he did know who she was and what she could do.
waaa, I also want so bad to know more about this character.
