A tall righty at 6'8", Fister's greatest asset is his command. He has only walked 11 batters all year in AAA in 106.1 innings pitched. Opponents are batting over .300 against him, but great command allows a pitcher to get away with that.
Doug may get hit pretty hard in the majors, but at least he will throw strikes. Nothing has been said about his role on the team, but my best guess is that he will relieve if needed, but may be in line for a start. Vargas was sent down to make room for Fister, so someone has to take the lefty's place in the rotation.
I would have stuck with Vargas, because I think he is better than Fister. However, Vargas and Garrett Olson are a little redundant. Fister's height and command give a little different look on the mound. I just think it's a very hittable look.