For the first 6 games, he was allowing completion rates that rivaled the best in the league, around the high 30 percent range. He picked off 4 passes, and many had him in the discussion for DPOY.
He then got hurt, missed 5 weeks (only 3 games due to the bye) and came back. When he returned, he went from giving up a completion rate in the 30's to giving up 2/3 of all attempts, as his rate jumped to about 66%. He failed to intercept another pass in N.England's last 12 games (including postseason). Out of all CB's in the NFL, Talib ranked dead last in yards per attempt from the opposing QB.
He obviously makes the secondary better when he's healthy and plays, but no one has any idea when he'll miss time, when he'll be 70%, or when he'll be like he was in the first 6 games of the season--at full health.
You seem to insinuate that he's some Superman CB by just being in the lineup, and that's hardly the case, as you can see from the above numbers. He needs to be healthy, and it's impossible to predict if/when that will happen. The fact that he has had a chronic injury for 3-4 seasons in a row (with other nagging injuries prior to that) doesn't do much to put anyone's mind at ease.
We all hope that he'll agree to stay here on fair terms, but some are acting like he's a once in a lifetime player or something.