Why do you play a freshman? Because you have to. Saban is old school, generally conservative and committed to "The Process". He played Hurts as a true freshman because he was the best option and he switched to Tua in the championship game for exactly the same reason.

Saban and his "process" operates similarly to Fuente and his "expected outcomes". Both value controlling the performance and getting the expected outcomes(i.e. leaving as little to chance as possible) from their QB. Hurts, as compared to Tua, gave Saban that option with very little chance of sustaining a loss during Alabama's regular season right up until the Auburn game. With Hurts you get a level of performance that is in a narrow and predictable range with the lack of a huge upside sacrificed to minimize the downside risk. With Tua, as was evidenced in the second half you have the potential for downside(think the interception)but, you also have a much higher ceiling with downfield throws. At the point when Saban put Tua in there was very little downside(for that game/season), he was already behind, unlikely to mount a comeback with the existing QB and staring a championship game loss right in the face so he rolled the dice. The really interesting part, IMO, and the real risk was what might have been the long term implications for the program had the decision failed that night. Lots of commentators talked about what a gutsy call it was on Saban's part and I agree but, not because it was gutsy for that game so much as what the long term repercussions of a negative outcome for that decision would have meant for his program long term.