The Steelers originally acquired Harrison as an undrafted free agent rookie out of Kent State University in 2002. After bouncing around a few years on practice squads, and in the now defunct NFL Europe, Harrison became a center piece of Pittsburgh's linebacking corp and helped the team win two Super Bowls in the 2000s.
Moreover, Harrison's amazing 100-yard interception return for a touchdown as time expired before halftime in Super Bowl XLIII is widely regarded as the greatest single play in Super Bowl history. Not only was it spectacular to watch, it was a game changing play that put the Steelers up 17-7 at halftime, rather than down 10-14 if Arizona had scored. Pittsburgh went on to win Super Bowl XLIII by a score of 27-23.
Harrison was the 2008 NFL Defensive Player of the Year, Steelers Team MVP in 2007 and 2008, and a five-time Pro Bowler.
While even most Steeler fans would agree that at age 35 next season Harrison wouldn't merit the salary he was due, no one in the game right now can play that position in the Steelers defensive system better than Harrison. Meaning no disrespect to Jason Worilds, who is Harrison's likely replacement, but Worilds is simply not enough to fill the void Harrison will leave behind.
Furthermore, it seems unfair that Harrison worked so hard to come back from an injury last season to get back to a level of peek performance -- only to be cut at season's end, while another (younger and better paid) Steeler linebacker, LaMarr Woodley was broadly criticized for not being in game form throughout the 2012 season.
Dear James, Steeler Nation will miss you!