First off, we didn't really play a 3-4 persay against Mizzou, we played a Nickel with one of the ends dropping back into pass coverage against the snap, making it resemble more of a 3-3-5. Second, the reason why we "stuffed" the run against Mizzou with the 3-3 is because Mizzou hasn't been effective running the ball all season, something EVERYBODY knew going into the game anyway. Third, dropping 8 back into a pass coverage zone is how you keep Mizzou's spread passing in check - it causes confusion with the Mizzou QB as he can't decipher what's going on downfield, forcing him to dump the ball for shorter gains.
There is a reason why we used this against Mizzou - because Mizzou is susceptible to it. Mizzou can't run the ball and it helps limit their downfield passing game. If we were to try running a 3-3 Nickel against a team capable of running the ball, for instance, Nebraska or Iowa, our defense would get gashed for 5-10 yards a play MINIMUM because our front 7 has enough problems stopping a power run game in the 4-3 formation. They would be MURDERED in a 3-4. Perhaps it's a formation we can work towards 4-5 years from now by recruiting for it, but running a 3-4 set as our base defense starting next year wold be a BAD IDEA, especially if we want any hope of making a bowl next year.
But I just love how, if a particular defense worked against a particular team, all of a sudden we should run it all the time without pausing a moment to think about why that formation worked against that particular team...