I was really surprised to read at Football Outsiders that the Green Bay Packers were second in usage of an empty backfield in 2010. While they weren't the best team in this formation, they were above average. When I think of an empty backfield, I usually think of an old run-and-gun style offense where the offense brings in an extra receiver instead of a blocker, and the quarterback had to get rid of the ball fast. And that doesn't seem to be the Packers style: leaving Aaron Rodgers without enough blockers.
But Football Outsiders did explain an extra wrinkle to what counts as an empty backfield. From FO:
Empty backfield, of course, doesn't necessarily mean no running backs in the personnel. The Patriots, for example, had Kevin Faulk or Danny Woodhead on the field for nearly all of their empty-backfield plays. The Bills usually had C.J. Spiller or Fred Jackson split out wide, and sometimes both.
Now that makes sense. You can see a couple examples of that in this highlight video from their playoff win over the Falcons. Early in the 1st quarter, John Kuhn is available to motion into the backfield as a pass blocker, but when they only rush four, Kuhn stays off the line of scrimmage and runs a short route. Later in the game, when the Falcons were down big and brought the blitz, the Packers left James Starks in for pass blocking.
It appears the offense easily has the flexibility to have a back block, or send him out as an extra receiver, depending on the defensive formation. And despite the high percentage of plays run in an empty backfield, it appears that this flexibility helps keep Rodgers out of any situations where someone comes after him unblocked.