Let's take a look at Texas's offensive stats before Bryan Harsin arrived, and since he's been here.
2010 — Year Before Harsin Arrived)
23.8 points per game
382.5 total yards per game
150.5 rushing per game (4.2 per carry)
232.0 passing per game
2011 — Harsin Year One
28.1 points per game
392.5 total yards per game
202.6 rushing per game (4.5 per carry)
189.9 passing per game
2012 — Harsin Year Two
36.1 points per game
441.0 total yards per game
176.0 rushing per game (4.6 per carry)
265.0 passing per game
How good has Texas's offense been in 2012? Good enough to play for a BCS National Title. Colt McCoy's final year on the 40 Acres, in 2009, Texas accounted for a field goal more per game (undoubtedly aided by a better defense), but passed for just about eight yards more per contest, ran the ball significantly worse (28.4 yards per game and 0.6 yards per carry worse) and put up both fewer yards (441-421.2 and fewer yards per play 6.4-5.6).
So what does that say to me? If Harsin's offense this year was paired with a better defense, the Longhorns could have been among the nation's top teams. Of course, none of that probably surprises you.
All the same, here are three reasons why Harsin leaving will hurt:
1) Offensive continuity. Texas returns both quarterbacks, its top three running backs, every receiver but one and all five starting offensive linemen. Those are a lot of reasons to believe that the offense could get even better in 2013, especially if Texas were to make a key addition or two along the way.
2) And that doesn't account for overall continuity. The thought-line employed by most is that Texas is gearing up for a potential big-time run in 2013 as a team. Like it or not, that's a big part of the reason that Mack Brown is getting another chance, and why other members of the staff aren't being pushed out either. So for one of the most important parts of the staff to leave is counter to the goal of circling the wagons for one last push.
3) Teaching David Ash. Harsin isn't a perfect offensive coordinator (who is?), but the one thing that he was beyond outstanding at was his duties as a quarterback coach. Ash came a long, long way this season under Harsin's tutelage, and the two maintained a relationship that, while professional, also stretched into friend territory. In short, Harsin "got" Ash, and was able to push him to new heights along the way. And while Ash is certainly a self-starter, bringing Harsin back was really the ideal way to continue his teaching.
In a couple of years, Harsin helped to change the mentality of the Texas offense, and every position on the team took a step forward, from quarterbacks to running backs to receivers to tight ends and finally the offensive line. Whoever comes in afterward, whether it's a simple promotion for Major Applewhite (which may make the most sense), will inherit a much better group than the one Harsin started with.