The real difference is defense. Morse can play a bunch of positions, but none of them all that well. Langerhans is a real good defensive outfielder, and as an added bonus can play a little first base. He should be up with the Mariners shortly, because he is the ideal replacement for Endy Chavez. Langerhans is even a bit younger than Endy, and I think will prove to be a better hitter too. The M's will have to do something to get Langerhans on the roster, but the easy answer would be to place Adrian Beltre on the 60-day DL.
Indeed, the Mariners offense has been difficult to watch this year, but this team is still on pace for a 15-20 win improvement over last year. Why? The defense. The pitching is better too, but thanks in large part to the defense. That is what makes losing Beltre so bad, and what made losing Endy Chavez so bad too. Langerhans fits in with the Mariners approach this year, and even if he does not hit terribly well, leaves the Mariners with only the one big hole at third base (well, aside from shortstop, but that has been a problem all year). Langerhans is not the kind of player that will transform the Mariners, but he is exactly what this team needed.