It's socialism. "Little Dave doesn't have any friends. Here, Johnny, you give Davey one, because you have four."
Never mind that Davey's parents don't bathe him more than once a week, whether he needs it or not.
Actually it is "socialization".
It really is better for children to develop with a group of friends, not just one. Usually within the group, the children will tend to be closer to one or two than the entire group.
Children who are able to socialize well within a group tend to be more outgoing, make friends/acquaintances better through life than if they were more isolated with only one "best friend".
It is true that a child who only has one friend, and only does activities with that one other child....does tend to become very possessive of that friend, particularly if the child does not have siblings. As we cannot control another's actions...eventually something/someone will come between that friendship, whether it be the family moves out of town, college, boy/girlfriends, etc. A child with just one best friend, will have a great deal of trouble adjusting to the status change of the friendship, regardless of whether or not the change happens when the child is young, or into their teens.
I learned this 40 years ago in a college Child Psychology class. I doubt that it has changed in that time.
What has changed, is that some children have isolated themselves into a "friendship" with a computer or MP3. This, to me, is much more harmful to a child than just having one best friend.