I very rarely say this, but mark my words......
The change of heart by the Big-12 and Big-10 about a playoff system just seems a little too abrupt for me not to wonder, "What's the catch"? I suspect that the catch is going to be that when they start to hash out the details, their proposed formula for setting the teams for the playoff will involve league or regional limits in some form or fashion. They are certainly not going to allow any possibility of the SEC having 4 of the 8 playoff teams, and they are certainly not going abide a system in which two SEC teams can play in the NCG. This is a trojan horse maneuver.
You may be right. To do that, they would be taking some of the power away from the polls. I'm not sure how popular that would be, but they are all tired of the SEC dominating college football. The idea of an automatic qualifier from 4 different conferences for the 4 team playoff would not be in the best interest of the game. But I wouldn't put it past the other 3 major conferences to try to push that through.