To be clear, I'm not completely opposed to some occasions or forms of direct ("violent") action. It can serve many purposes
1. First and foremost, the anarchist ideal cannot co-exist with the current structure of society. Any form of change will eventually come to blows. I'm not exactly an insurrectionist. I believe that culture and society are deeply tied. People live in the best way that they know how. If a handful of insurrectionists managed to tear down the established order, then that would create an opportunity for change. Unfortunately, it would almost certainly lead to an immediate recreation of the exact same society as before, that being the society that the majority of people already know and understand. There needs to be a widespread general understand and acceptance of an alternative within the dominant culture for change to be possible. This is a long-term goal that direct action can realistically only play a very small and careful role in. However, in the end there will be many, especially among the ruling class, who oppose change and will refuse to co-exist with a way of life different from theirs. Current society is predatory, after all, and that is the root problem. If anarchists could simply band together and make a place for ourselves seperate from the rest of the world, we would, but any large enough project would be destroyed from the outside, just as indigenous cultures around the world have been almost completely eradicated. If a switch is ever made, there will be violence involved.
2. One of the ills of modern society is the feeling of powerlessness and apathy among the average person. Direct action is one powerful way to re-establish a sense of independence and volition.
3. The resulting PR may make a bad impression on most, but those same people would probably never even know of the existence of anarchists in the world any other way. Still others will have an opportunity to recognize that they are not alone in the world. I had determined myself an anarchist by the age of 15, but didn't actually know there were signification numbers of people with similar ideals or even organizations until I was in my mid-20's. I felt very very alone and hopeless, until I started reading up on events like the Battle in Seattle. Through the perspective offered by anarchist organizations testing their strength, I realized that I am definitely not alone, and that there are actually enough of us out there to make a difference when we band together and play our cards right. We need to let the world and our own potential allies know that we exist, and I don't think there's any better way to do it.
4. Directly harming organizations that are seen as especially oppressive is a legitimate tactic. This shouldn't even require explanation. The only debate regarding this is how realistically effective this is vs the risks associated. Yes, major banks are really evil and smashing up their offices costs them a lot of money and stuff, but does it really make enough of a dent to matter and can we afford to risk the jail time when there are so few of us?
To put this in perspective, I was really disappointed to hear about the recent sports riots in Vancouver. I thought that was pointless and counter-productive. I could understand the reasoning for action at the Olympic games (heavily associated with deeply nationalistic sentiments among other things), but still find it very questionable basis for causing trouble. I don't exactly condemn it, but I sure wouldn't participate. G20 summit? Absolutely a time to make your presence known. If I didn't have a family to care for, I would be there.