I've been reading many analysts argue there is precedent in letting only conference champions in the playoffs. Granted, only conference champions have made the playoffs this far, but IMO there are several factors that supercede the conference championship.

1. The loss column.
The gold standard measure of success in college football. Within the major conferences, teams with 1 loss a jump undefeateds, and teams with two losses rarely jump teams with no losses (exception is Bama).
2. Head-to-Head.
Sparty didn't get in last year because they won the conference championship guys. It's because they beat us Head-to-Head. If the loss column is equal there isn't a stronger argument than the two teams have already settled the dispute on the field.
3. Strength of schedule/quality wins.
Ohio State's schedule last year was probably the weakest it's ever been. Two years ago TCU and Baylor both played schedules where it seemed the only quality opponents were each other. And people forget this, but Briles openly said Baylor had no plans to ever beef up their schedule after the committee said it wanted to see teams play better games. Hmm. Wonder why Baylor was ranked behind TCU till the final rankings came out.
4. Eye Test.
Do you guys remember the Wisconsin game in Indy? Damn, we looked unbeatable. And the committee seemed to agree.
5. Big name schools. Big time money.
This one's harder to prove but brand matters. If you think Washington is getting in ahead of OSU if there is ANY reason to leave them out then I will laugh in your face.

So I guess my point is that although conference champions have correlated to playoff teams it does not infer causation. There are, I believe, stronger arguments as to why the committee has chosen the four teams they did.