While teams like Oregon, Kansas State and Alabama play a few out-of-conference foes each year that are much lower quality than these teams should be playing, there are plenty of quality cross-conference matches that occur every single year. Just during rivalry weekend, here they are:
South Carolina vs. Clemson
Georgia vs. Georgia Tech
Florida vs. Florida State
Vanderbilt vs. Wake Forest
Notre Dame vs. USC*
Clemson, South Carolina, Georgia, Florida and Florida State all have BCS hopes riding on the results of rivalry weekend.
Not every year gives us a rivalry weekend with BCS hopes on the line for more than one team, much less five teams across three games.
Every year, the ACC and the SEC clash on rivalry weekend, and it's almost always a fireworks show. Streaks come and go, and so do champions. BCS at-large bids are won and lost in some of those games every year, even as conference championship runs remain intact.
For instance, if Florida State loses to Florida, the Seminoles would still be the favorite to win the ACC title. However, if the Seminoles won, the Gators would likely be knocked out of the BCS picture.
Quality matches that reach across conference barriers don't happen that often outside of rivalry weekend. That's what makes the weekend so special.
*Notre Dame vs. USC would be cross-conference if Notre Dame were to join a conference.