Here are some stats for you from 2007:
Top 10 offenses:
1. Tulsa - 108th in defense, 10-4 record
2. Texas Tech - 45th in defense, 9-4 record
3. Hawaii - 34th in defense, 12-1 record
4. Houston - 46th in defense, 8-5 record
5. Missouri - 59th in defense, 12-2 record
6. Louisville - 84th in defense, 6-6 record
7. Oklahoma State - 101st in defense, 7-6 record
8. Kansas - 12th in defense, 12-1 record
9. Nebraska - 112th in defense, 5-7 record
10. Oregon - 60th in defense, 9-4 record
Top 10 defenses:
1. Ohio State - 62nd in offense, 11-2 record
2. USC - 29th in offense, 11-2 record
3. LSU - 26th in offense, 12-2 record
4. Virginia Tech - 100th in offense, 11-4 record
5. Pittsburgh - 108th in offense, 5-7 record
6. Auburn - 98th in offense, 9-4 record
7. West Virginia - 15th in offense, 11-2 record
8. Oregon State - 78th in offense, 9-4 record
9. Clemson - 52nd in offense, 9-4 record
10. BYU - 25th in offense, 11-2
As you can see, having a great defense doesn't guarantee a great season. Neither does having a great offense. To have a really successful season, you have to be pretty good in both. There are only a handful of exception for teams that had success while sucking at one or the other. Tulsa, Louisville, Oregon, and Missouri all managed to have decent to pretty good seasons with good offenses and mediocre to bad defenses. Virginia Tech, Ohio State, Auburn, Oregon State, and Clemson all managed to have pretty good seasons with great defense and weak offense. Pittsburgh and Nebraska prove that being great and one side of the ball and very bad on the other can still get you a losing season.
I'll also use our team from the early 90s as an example that good defense does not necessarily equal wins. We had the No. 1 defense in the country, and we went 5-6 because our offense sucked.