opponents are "better" and start looking at score differentials, which are often useless. For instance, Boise State in 2006, before playing in Fiesta Bowl, only beat San Jose State by 3 and Wyoming by 7. Their only decent win prior to the bowl was Oregon State.
Oklahoma had a much tougher schedule. Was Oklahoma therefore the "better team"? In a 4-team playoff should they have left BSU out (which obviously they would have)? Or would it be better to see if an undefeated team can remain undefeated, regardless of SOS? If they suck they'll get blown out in the first round of the playoffs.
There have never been more than 5 undefeateds in a single season, btw. It's an accomplishment that should be rewarded with at least a chance in the postseason.