"There's an upside and downside to everything, but if I were running the show..."

Holding companies control a large portion of the marketing communications industry's resources (agencies), as well as a sizable portion of client revenue and relationships. What do you see as the "upside" (i.e. advantages) and value of a holding company relationship for a client organization?And conversely, are there any "downsides" (i.e. disadvantages) or limitations that you see in a client having a relationship with a holding company?And, if you were a Chief Marketing Officer (CMO) in a client organization, how would you structure your agency roster and relationship?