I stopped using that after I had an incident where one of my LBs was covering a WR, because the opponent was playing a TE in the slot. It resulted in a long TD pass for the CPU because of it.
The play calling screen said that the offense was in a 2 WR, 1 TE, 2 HB formation and I chose a 3-4 defense (Ravens). When they lined up, I saw that the TE was aligned in the slot position with my best CB covering him (their best WR was apparently out because of fatigue or whatever, I don't know, but I don't understand why the AI decided the top CB should cover the TE?) and my weakside linebacker (J. Johnson - 70-something speed) was covering the outside WR. The CPU snapped the ball before I could prevent or limit the damage. Needless to say that I haven't used defensive assignments since.