If someone is behaving in a way which is causing them to be socially ostracized, bullying is arguably only going to make the problem even worse, and in my own observations, it invariably does. People can argue that the victim "brought it upon themselves", but this doesn't suddenly make the bullying constructive; it almost never teaches the victim anything, it just makes their life miserable, while also working to cultivate an unwarranted sense of superiority in the bullies which is likely to discourage much self-improvement on their own part as well.
I can see no positives to it, especially since many of the victims of bullying are targeted for reasons which have nothing to do with things that they can or should change, but everything to do with the things they can't (like disability, height, ethnicity, etc).
I definitely agree that much of our anti-bullying approaches nowadays are flawed, although the problem, I think, isn't so much that people are being "sheltered" from bullies, but that authority figures are failing to take into consideration the social dynamics which cause bullying. When people are behaving in a way which invites social ostracization, the constructive thing to do is to take them to one side (preferably from a young age), and teach them how to improve their behavior so as to not be ostracized. That way, they can engage in normal social interaction like everyone else, and everyone (in theory) can be happy.
