Personally I love them. The Carolina Challenge Cup, the PanPacific Championship, the Disney World Pro Soccer Classic, even La Manga with the Sounders!
In fact one of the things I love about Major League Baseball is the annual tradition of Spring Training. It would be nice to see MLS develop a history and tradition with these affairs. Every year a team's supporters should know where they will train, and where they will be for these mini-tournaments.
DC and TFC are nearly locked into the CCC, and their fans travel for the first look at trialists. Any club could establish this habit, those connections. They could work with their travel partners and likely even make a buck or two.
With a 20 team league it might be nice to see 4 or 5 of these scattered through the sunbelt every year. Hundreds of thousands go to Florida and Arizona during Spring Training, there is no reason this can't happen in MLS.