In the history of MLS, there have only been four Designated Players who have not been midfielders or forwards: Rafael Marquez, Torsten Frings, Frank Rost and now Omar Gonzalez. Marquez and Frings were converted to midfielders and Gonzalez is certain to remain at center back as Rost was always going to be in net.
Designated Players have not been the best bet for teams since the so-called Beckham rule was instituted in 2007. Marquez, because of his infighting with the New York Red Bulls and lack of production (and ability to stay on the pitch), is considered to be one of the worst Designated Player signings ever. He scored just one goal in MLS (and none in his final two seasons which consisted of a total of 34 matches). On a list of 22 multiseason DPs from 2007-2012, Marquez ranked 16th with a career Match Impact Rating of 20.3. Frings scored twice in 2012 but only accumulated five points in two MLS seasons and was 17th on the list of 22 multiseason DPs with a MIR of 13.9. As for Rost, he was limited by injury during his sole MLS season, won 35.7% of his matches (including playoffs) and saved 69 percent of shots he faced.
Gonzalez, unlike a good number of MLS midfielders, can say that he has scored at least one goal in each of his five MLS seasons and he has two MLS Playoff goals. Gonzalez would have ranked 20th on the list of multiseason DPs with a MIR of 9.6. However, he has put up a MIR in the double-digits each of the last three seasons (11.9) and is having an average offensive impact for a defender in addition to his stellar defensive work.
The LA Galaxy still need a distributing midfielder to pick up the slack from the departure of David Beckham. However, being able to lockdown the league's best defender who can also chip in the occasional goal was a no-brainer. If this is Landon Donovan's last season in MLS (and there's certainly a chance it might be), the Galaxy are going to have plenty of money to find that midfielder they've been missing.
Another way to look at the situation is this: if Orlando City SC had said they wanted to bring Pepe instead of Kaka to MLS in 2015, would it have been met with disdain or would it have been a source of excitement?