I can understand not wanting to present it as a bunch of flashbacks. That format works better in a book than in a movie. But one of the best points about the book is meeting all these characters and getting their stories. If Pitt is the guy who goes from location to location interacting with them, it could really work.
But if they're going to abandon that, then what is the point of turning World War Z into a movie? World War Z is different than the usual zmobie movies. Leave it to Hollywood to crush any originality in order to make it conform to the same old shit.