Defense wins championships, and d-lines dictate how good your defense can be. Look at the standout defensive linemen over the last few champion teams, pro-bowl appearances in parathesis if applicable:
2006: Colts: Booger McFarlend (im astonished he's never been to a probowl),Robert Mathis (50 tackles, 15 assists, and 9.5 sacks dont get you in?!),Dwight Freeney (3)
2005: Steelers: Casey Hampton (3), Aaron Smith (1)
2004: Patriots: Richard Seymour (5), Willie McGinest (2)
2003: Patriots: Richard Seymour (5), Willie McGinest (2), Ted Washington(3)
2002: Bucs: Simeon Rice (2), Warren Sap(7)
2001: Patriots: Richard Seymour (5), Willie McGinest (2)
2000: Ravens: Sam Adams (3), Rob Burnett(1),
and i probably forgetting some key guys...and i realize pro-bowl appearances arent the greatest indicator of talent but thats still a pretty good who's-who list of great d-linemen in the current era of the game.
I think the point is that you can win a Super Bowl with Trent Dilfer as your QB, Antowain Smith as your RB, Troy Brown as your best WR, or Kelvin Hayden as your 'big play' CB, but you arent going to find many (if any) successful teams that didnt have some studs on the d-line. And the great alltime teams had a bunch. This game is won and lost right in the trenches, and the deck is heavilly stacked in favor of the offensive line. So your d-line is absolutely critical. To be honest im surprised teams dont vie even harder for the top prospects at those positions.