I don't know how powerful he was actually, but he definitely was not as weak as he is in game.
There is absolute no use for him at the moment, even some could argue that Jedi Consular has more use than him. His kit is more than outdated.
With that said, he was second in command only to Yoda, which says a lot.
Also, his technique in lightsaber dueling was unique and really powerful. We saw how effective he was against probably one of the most powerful characters in the story, Emperor Palpatine. Palp destroyed every single jedi that entered his office, other than Mace.
Lastly, a character that is indeed played by Samuel L. Jackson, hints that he is probably somewhat of significance to the story. Why would you pay an actor of his caliber to play an unimportant character?
Don't make sense!
