I think Shawn Fonteno did a great job voice acting him, but I'm question if he was needed for the story at all. I, like everyone else, was excited to "return to the hood" and re-live some San Andreas memories, as well as see crime from three perspectives, but it didn't work out quite well.
While Michael and Trevor have deep ties with each other, Franklin's relation to the other two is shallowed and feels forced. Also, he doesn't seem to have a lot of motivation to stick around with two middle-aged guys doing profit-less errand jobs for the FBI that could cost him his life while he already has a wealthy house in the hills.
He also complicates the ending, because it waters down the tension between Trevor and Franklin by making it less personal. He always felt like the odd one out of the three and just tagged along anyway.