In an era dominated by offense, this is an old adage that's lost all meaning. Last year's Baltimore Ravens (historically known for their D) ranked 17th in total defense and in the playoffs, they were forced to win two 30+ point shootouts (including the Super Bowl). The 2011 champion New York Giants actually got outscored in the regular season 400-394. The 2009 Saints...you get the point.
Offense will reign supreme for the foreseeable future because that's what fans want. Nearly every major rule change over the past decade favors offenses in the NFL, which only stands to increase with the awareness of head injuries. That's why the New Orleans Saints and New England Patriots of now, the Indianapolis Colts of the aughts and the "Greatest Show on Turf" are (or were) year-in, year-out Super Bowl contenders. It's also why we've seen five quarterbacks eclipse 5,000 passing yards in a season over the past five years.