Back when I played, the Soldado play would probably have been flagged as off-side just because Eriksen was behind the defense, but the run Soldado made - from off the back line, is exactly the run we were taught to make, particularly on free kick situations. The players who were forward, and on the defenders shoulders, so to speak, would be moving toward the ball, while a midfielder (presumably unmarked) would be making the run up field - to catch the defenders going in the opposite direction trying to pull the off-side trap.
It is a little funny that those saying this is unfair to the defense, when it is the defense who is trying to pull some trickery by stepping up before the ball is played. Even if Eriksen had stepped up with the defense - keeping him in an on-side position, the defense was getting burned by Soldado's run - thats a fault of the trap, not a fault of a player like Eriksen being in an off-side position.
Where I get frustrated with the rule is closer to goal where you have a player in an off-side position who can make a play on the ball, and freezes a goal keeper who has a split second to determine what that player is doing, and as a result the goalie makes a poor attempt on the ball. Inside the box should be an automatic determination that the player is part of the play.