Also a pretty common misconception.
http://www.footballsfuture.com/phpBB...c.php?t=328837
Check that out. It's a collection of statistical data on teams who have switched to a 3-4 defense over the past 10 years. Most teams saw a significant improvement in overall defense in their first year running the system, whether they switched to a full 3-4 base, or ran a hybrid system.