I think that the dangerousness of having a best friend really depends on the person.
For example, when I was being 'homeschooled', it was really more like a private school with a few kids in the class. I had a best friend because he would consstantly get frustrated with his school work, and my natural inclination was to help him understand his work. We had a really good friendship, and since the other kids were my cousins or close family friends, I felt like I couldn't be exclusive with just my best friend. We never had any real problems, except that he moved away, but there's really nothing I could've done about that.
In public school, however, it's not a sheltered environment full of people that treat eachother like family. It's actually quite harsh and conflict filled. Kids feel like the way to survive is to find friends that will stick with them and keep them 'safe.' Sometimes you get kids who just really like eachother, and their friendship is natural, so they consider eachother best friends. From there, it can go in a lot of different directions. Some things I've noticed from experience:
-Some introverted kids who have trouble socializing and are lucky enough to have a best friend might feel like they need to do everything in their power to keep their friendship. If an introverted child is made to spend time with others to 'prevent cliques from forming,' it usually does more harm than good, as the introvert's best friend is the one person they can trust, and it takes time for them to open up to others. You can't force someone to be more social than they feel comfortable with. I've seen too many teachers and parents try to make people like eachother. It's really quite naive.
-Other kids actually do form cliques. They're usually the ones with the high standards and low opinions of other people for a variety of different reasons. They form alliances to get a sense of power over people, and it usually leads to exclusion and bullying, which I think adults should definitely get involved with if it's too much for their child to handle on their own.
But basically, it's definitely context-dependent, since every child is different.
So I don't think it's dangerous for a kid to have a best friend, as long as the fact they have a best friend is healthy. It can be a good learning expereince to have frienships that don't work out, or arent' perfect, as that part of life is inevitable. I think parents should be willing to talk to their kids about their friendships, be aware of the kinds of people they're spending time with, and give them helpful advice, but let them make friends in a way that suits them, like the article says.