Because all I have are anecdotes, but all of my anecdotes are on the side of shared knowledge being heavily biased towards safety and consensus building demanding a lot of time. That happens even between two people that mostly agree, and only increase with more people or different opinions.
Of course, some times a bias towards safety is exactly what you want. But on informatics projects, that happens very rarely.
Once in a long while two people can have complementary ideas that make them faster. But it tends to happen more often when one is the clear owner of the project and the other is advising. Shared ownership seems to almost kill those chances.
