The New York Times looks at both sides of the debate. On the one hand, some psychologists think kids need to learn to network and form diverse interests with lots of diverse friends. On the other hand, other psychologists believe that having the close intimate bond of a "best friend" is important for later adult relationships, and is preferable to having a lot of superficial attachments.
My take? Simple. Can we not have this debate at all and stop trying to micromanage our kids?
UPDATE: Perhaps I need to expand on this.
I think the tendancy for psychologists to cookie cutter today's youth with proclamations of what is "healthy" or "unhealthy" can, if taken to extremes, be just as detrimental to kids as the things that psychologist purport to solve. Should children be allowed to have best friends? The answer, stupid, is… it depends on the kid. There are way too many other factors bearing on a child's social health (family being the biggest one)… so perhaps some kids need a best friend, and others don't. There is no "one size fits all" answer. And I believe that instinctively, each kids is probably aware of his or her needs, even if they can't articulate the root cause of why.
So if little Johnny gravitates toward having a best friend, it's probably because that's what he needs. If little Janie is the type to have a wide circle of friends, but not a single "best" friend, well, that's probably what she needs. But let's not sit back with our clipboards and move them around like little chess pieces under the believe that "having a best friend" is either a "psychological good" or a "psychological bad". Let's trust kids to follow their instinctual needs, and let's not assume that they ALL will have the SAME need.
The whole debate reaks of Machiavelli.