Whether Seattle was one of the teams that Toronto was talking to or not, we do know now what the price for Dwayne De Rosario was. The amazing talent, probably the only one who could challenge Landon Donovan for best in MLS over a significant period of time, was dealt for Toni Tchani, Danleigh Borman and next year's first round pick.
Tchani is only in his 2nd season, but is a solid talent with upside. He has 18 starts in 29 appearances for New York Red Bulls. Borman has 35 starts in 59 appearances in his two+ seasons with Red Bull. Neither is lighting up the stat sheet, but in all the New York club gave up two not-quite starters in their pre-peak, plus a likely starter in their next draft pick.
And of course they used their third DP slot. The move takes up enough cap space to prevent a 3rd DP from being signed.
For Seattle this kind of deal might have meant someone like a Patrick Ianni, Brad Evans and the draft pick - if not MORE. It would have ripped up the depth of the team, but added high-end talent. In essence 3 starters for one Best XI. Generally, that's a good deal for the team getting the best player.
Unless you already have a great player with the similar style and role to the man being acquired.
For Seattle, and Los Angeles Galaxy DeRo is redundant. Fredy Montero and Donovan are that type of player. In the Sounders case, with those changes they would now be counting on newcomer Erik Friberg in nearly 100% of their 50+ matches. They would still have Alvaro Fernandez on the bench, and the band of three would be a potent Steve Zakuani, Montero, DeRo.
The splash would have been great. Seattle would have three players capable of a double-double on the season. That third potent attacking player would prevent many of the techniques being used to stifle the current two prong attack.
It would also shift Seattle from being a constantly good team by structure to one that goes into a win-now roster churn mode. Both philosophies work, both can win titles. The structure that New York is committed to is one that will have to involve snagging 4-8 starters every single off-season and hoping that they can figure out how to work together. The structure that Seattle and Real Salt Lake have taken of low churn involves constant development from within while bringing in 1-3 starting quality players every season.
A move for DeRo would have been a great move for a one season run at the Supporters Shield and MLS Cup. The low churn model is instead designed for success in the US Open Cup, CONCACAF Champions League and runs in the Playoffs.
DeRo would have been nice, but it wouldn't have been consistent with Seattle's aspirations.