According to Pro Football Focus, Roethlisberger led the NFL in targets to inline tight ends in 2013 with 80. His 78.8 completion percentage (63-of-80) was the third-highest in the league behind Phillip Rivers (81 percent) and Carson Palmer (79 percent).
Miller was third on the Steelers with 78 targets after having missed the first game of the year due to a torn ACL suffered in Week 16 of the 2012 season. He probably wasn't even 90 percent healed from the injury until well into the year.
All reports suggest Miller is back to full strength now. An often overlooked aspect of the 2013 season was the injury suffered by TE Matt Spaeth in training camp. The veteran missed 12 games after having signed with the team, largely due to Miller's injury. The instability of the position (they lost David Johnson after Week 5) led to struggles on the offensive side of the ball.
Johnson is in San Diego now, but Miller and Spaeth are both healthy. Roethlisberger's passing numbers to inline tight ends are good, but it's their presence in the running game that was missed more last year. Getting both back full-time will provide a boost in both phases of the game.