I go back and forth on this. On one hand, I think that every conference's best team should have a chance to win a national title. On the other hand, I think the odds of the 6th or 7th ranked team (or "best" team, whatever you want to call it) reeling off three straight playoff wins is extremely remote, so in a lot of cases the quarterfinal games might not be all that compelling. Maybe we're just in a time where Alabama's dominance kind of skews things a bit, I don't know. But it's hard to compare to DII and DIII (or basketball) because the logistics of staging a game with 80,000+ people vs one with 10,000+ people isn't comparable. I just think the solution to going to eight teams isn't quite as simple as it sounds and at the end of the day in most cases you'd probably be adding another round of games but getting the same result in the end.
So, hypothetical situation. The P5 conference champions are all undefeated as is Notre Dame. 4-team playoff... who gets left out? You really gonna say that an undefeated team rank #6 has little to no chance of winning the whole thing? What are you basing the merit of each team... SOS? Human decided rankings?
The problem with any sort of ranking system is that there's only so much extrapolation you can really take from numbers. Why? Because not enough the teams that are in direct competition for playoff spots play each other. Maybe the SEC is way better than all the others... in which case the argument is made that Georgia would likely beat everybody else and possibly even beat Bama in a rematch. In the Undefeated 6 scenario, Georgia could make a real case for the #7 spot.
But maybe the SEC isn't as great as everybody makes it out to be. How would we really know? The SEC largely plays... other SEC teams. That's how conferences work. We only know how good these teams are against their own conferences (with the occasional premier matchup in non-con early on).
So, who is to say that a Big12 champ with the same record as the Big10 champ is actually more worthy of a playoff spot?
Nope. If you win your P5 conference, you should get the opportunity to compete for the National Championship. Period. And if you aren't in a P5 but go undefeated, maybe you should get a shot. Why not? How does that actually hurt anything? It just gives another opportunity for some really interesting games.