I'm not sure I buy the premise to begin with that there are "great leaders". At least, as far as group tactics are concerned. There are characters who know tactics very well and try to share that knowledge with those around them, but there is just too much autonomy for each individual character for there to be one guy who somehow "makes everyone better" by virtue of his leadership skills. He can coach them before hand, he can draw up a game plan, but if his supporting cast sucks then it's all going to fall apart once the fighting starts.

That said, I think there are some characters who, through sheer power, manage to make groups succeed that would have otherwise failed. Lariya comes to mind. Having someone like this around at least raises the confidence level of that person's allies, even if it doesn't make them any "better".