If the Sounders keep Alonso they are perfectly set-up to play with three central midfielders. If they do not have him, than Erik Friberg provides versatility in the search for a replacement.
For the six years of the Seattle Sounders existence, Sigi Schmid has almost exclusively used a 4-4-2 formation. Much of that time, that choice has come down to the fact that Osvaldo Alonso is one of the most offensively and defensively dynamic players in MLS. His skill on both sides allows the Sounders to effectively fill the roles of two players at once, freeing them up to add shift other players duties to suit their particular skillsets better.