Stewart was a victim of terrible play-calling in Pittsburgh. See http://www.post-gazette.com/steelers/20030806snoter0806p7.asp for an example.
At the beginning of the season, Mularkey was more interested in "spreading the ball around" than he was in trying to win. Than, Kordell went out and Maddox went in. At the same time, Mularkey opened-up his play-calling and forget about that spreading-the-ball-around nonsense.
Besides, Stewart is a huge improvement over Miller, who not only couldn't scramble, but got injured every other play and rarely threw the ball further than 4 yards.