I think it's a matter of the athleticism/confidence of the individual players and the technique that is coached.
It's obvious that if a DB does not have confidence in his ability to stay in phase without having eyes on the receiver, then he will be hesitant to turn his head for fear of getting burned. On the other hand, an overconfident DB can be just as bad. Hadden's recent play has been less confident and it's caused him to miss some opportunities. While a guy like Slaughter played with a ton of confidence last year and had mixed results. We all remember him making highlight plays like the high-point interception against Kentucky last year, but some receivers were able to use his overconfidence to their advantage and make him bite on double moves and such.
I recently read some coaching material from a DB camp that specifically teaches DBs to keep their eyes on the WR and read his cues in order to react to a ball in the air, sometimes you'll hear this referred to as "playing through the receiver's hands". It's a constant battle between DBs and WRs trying to read one another's cues to anticipate what their next move should be. The best receivers can trick a DB with their eyes and hands into not reacting to a ball in the air. I don't know if Martinez explicitly coaches his guys to play this way, but it definitely seems like some of them are attempting this technique.
It all looks very simple from the comfort of your couch on saturdays, but these guys are playing constant mind games to gain advantages.