Not necessarily. Michigan State was 11-1 and didn't go to a BCS game last year. If I made the Mountain West a BCS conference, and they only got 1 team into the BCS, then I could have a 10-2 or 11-1 team playing in the Armed Forces bowl against a conference USA number 3 team.
Part of the fun of conferences is the conference matchups in the bowl season. Again, if I moved Texas, Oklahoma, Texas A&M and Nebraska to the Moutain West (for example) and Texas, Oklahoma, and Nebraska all won 10 games. One of those teams would get to a BCS game, but the other teams would be playing in crap bowl games against crappy teams instead of playing in new years bowl games against other good teams from good conferences.
And if I shrunk the Big 12 down to four teams. A 7-5 Kansas team (which might be the Big 12's second best team in this scenario) would be playing in a New Years Bowl game like the Cotton Bowl against a top rated SEC team.