It's true, at this point he has almost no trade value - but as Pookie points out that's more becaue he's getting older and not worth the cap hit. It's not unlikely there was some league involvement in the trade - they own all the contracts and approve all the trades and have been known to make decisions based more on marketing than anything else and DeRo in Toronto was good for marketing - just like McBride in Chicago was good for marketing.
One thing this really shows, though, is the importance of management in a salary cap league. The teams with better managers, who can get the most out of "whiners" and players who feel they should be getting paid more or be in better situations and who can manage personalities will win a lot more games.
And that's why things are looking up for TFC - as long as the decision to keep or cut DeRo is completely that of the soccer guys and marketing isn't taken into account at all.