On offense, you are not allowed to switch to the receiver to make a catch. Once he catches the ball, you can control him. On defense, you start the play as one position (whatever you choose), but once the play starts, you are not allowed to switch players.
As A&S says, it allows for the true talent of the players on the field to decide the outcome, not YOUR talent level with a DB or WR.
Even using the no switch rule, I've gotten a few picks with my user controlled safety.