The article points out that many protests in the U.S. went smoothly through the practice of police and protest organizers meeting and jointly managing protests, but that this practice fell into disuse after the 1999 Seattle WTO meeting in which protesters violated the negotiated terms and police responded with violence.
While some recent (and ongoing) protests have turned violent, many didn't. In the coming months we'll have time to do a postmortem. I strongly suspect spontaneous protests without organization will be found to have the most potential for violence, while those with organizers committed to self-policing and, ideally, cooperating with police will be found to have fared much better.
Individual people may be intelligent and responsible, but crowds have their own rules of behaviour and need to be managed. Protests are more dangerous when unplanned or when their organizers give no thought to self-policing.
There will always be organizers who want violence because it reliably brings press coverage and attention to their protests, but social media is also creating new problems. Coordinating a large number of people to show up at the same time and place used to take considerable planning and effort. When you have to work hard just to get the even to happen, why wouldn't you plan how it will unfold as well? Now a couple of tweets or posts on the right reddit subs will suffice. How can police meet with the organizer of a protest when it's really just some dude who had a lot of social media followers and might not even bother showing up himself?
1. Should there be police at all?
2. If so, can an institution staffed by these particular people ever function correctly?
The more effectively the protesters self-police, the more powerful an argument they make that you could improve society by disbanding urban police departments and letting the protest organizers assume de facto responsibility for maintaining the social order.
Of course, the other end of the violence spectrum is an effective argument, as well.