Avoiding too much of a good thing is normally sage advice, but this rule is often not apt when describing how to build an NFL defense.
Take the case of the Pittsburgh Steelers and linebackers over the past quarter century. A quick use of the Pro-Football-Reference.com draft finder tool plus a count from the 2011 draft shows that since 1985, the Black and Gold have selected 43 linebackers. That total is tied with the Tennessee Titans for the most in the league in that time frame.
The argument can be made that Pittsburgh drafted a high volume of linebackers because they are one of the few teams to have operated a 3-4 defense during that entire time period, but in many cases, the Steelers drafted linebackers even when their roster was already stocked with high-caliber talents at that position -- like in 1987 when they drafted Greg Lloyd and Hardy Nickerson despite having quality young starters in Mike Merriweather and Bryan Hinkle.
It's an unorthodox approach, but it has worked because Pittsburgh's coaching staff keeps getting the most out of the overflow of linebacker talent the front office sends its way. The defense is seemingly built around that constant depth.
Adding Fairley might seem like an unnecessary luxury to some, given that the Lions already had 2010 Defensive Rookie of the Year Ndamukong Suh on their roster, but a review of the game tape and numbers shows how head coach Jim Schwartz and defensive coordinator Gunther Cunningham will be able to get the most out of this talented pair of DTs.