In a my 12 team league, most weeks the defense playing the Vikings or Texans won't be available. And at $5 per pickup, it's not cheap either.
As for the rest of it, as stated, in my league, defense is a significant player. The differential between a good one and an average one is easily 5 or 6 points per game on average. I don't need another RB, WR, or TE. But I do need a defense. I have Cadillac, Willie Parker, Thomas Jones, Chad Johnson, Joe Horn, Darrell Jackson. Of this group, I need only start 1 RB and WR, with 2 flex. Didn't feel like waiting around for KJ to maybe come around so he could maybe put up numbers close to 2 of the RBs I already have, all the time losing 5-10 points a week from my crappy defenses. It's a 12 team league and even with trading KJ, I still have the deepest roster at RB/WR in my league.