A couple of thoughts here:
1) There was a lot of talk in Phoenix about starting Leinart this week - let Warner come off the bench if needed
2) Second even if Warner starts, he does have games where he is awful....holding on to the ball too long and giving up a lot of sacks and interceptions.
Lastly, most FF leagues don't penalize DST for points scored or yards allowed > they get points for sacks, interceptions, fumbles recovered and touchdowns from the defense or special teams. In the high stakes competitions there are very small points for the opposing team having a low score and nothing for yards against and no negatives, so the rankings should look a little different and Tennessee, at home vs Warner coming off a concussion or Leinart playing are a decent play.
Unfortunately there are NO FF sites that explicitly describe what their DST rankings are based on.....