There's a line of thinking that developed out of the Seattle Sounders community called "trophies not friendlies" and has been trumpeted by the fine folks at Sounder At Heart for a long time now. The thought is that MLS needs to focus on enhancing its reputation and significance in order to gain media coverage, prominence and popularity.
Another theory is that of international friendly exposure. By bringing in "giants" from throughout the world, usually from top leagues in Europe, MLS gains exposure it would not otherwise have. It also adds coverage from news outlets that wouldn't cover MLS normally, or at least at the same level as the friendly. Moving a middle of the season MLS league game would, under this theory, be able to be moved to support a big time friendly without being "unethical."
There's no wrong or right answer in the end, because everyone's perspective on how MLS can improve itself in the short term for the long term is different. With Real Madrid coming to Philadelphia after the Philadelphia Union and the Sounders agreed to push back a game at Seattle's QWEST Field, both sides of the argument came to light on social media networks and other areas of Union conversation.