The set of international friendlies that MLS hosts every summer is successful in terms of revenue. But in terms of growth, I would argue that's not completely beneficial to the growth of the league, probably more detrimental or at least negligible.
Truth be told, playing the likes of Manchester United, Chelsea or Everton won't help the league grow. It's not going to capture the fans that have ignored or looked down at the league for 10-plus years.
Most of the time these friendlies aren't even played in the team's stadium, but a much bigger one—which eliminates the intimacy and environment that usually exists in the league's soccer-specific stadiums.
What makes the league grow are the fans themselves. They are the expanding force for the growth of the league these last seasons. Fans bring friends, who more often than not become fans. That same trend carries on with the next friend, and so on.
Personally, when I take friends to their first LA Galaxy match, it's never LA vs Manchester City, or LA versus whatever European team decides to do a preseason tour in the states.
The matches I take them to are the matches versus Seattle, D.C., Chivas, San Jose, etc. Those are the matches that truly display the passion that MLS fans have.
They can enjoy and admire the environment of MLS that is unknown to most unless one actually steps foot in a stadium.
And have they been disappointed?
Never. I always get the same reaction.
"Damn, is it always this crazy?"
"Uh....yes. Welcome to MLS."