Id never rule out violence, especially in terms of personal self defence - but that is up to those doing the demonstrating, not for anyone sat on a bulletin board to demand.
My thoughts for what the are worth, is that a long campaign of non-violent civil disobedience and direct action is far more likely to be successful if it holds out against using violence at all, or for as long as physically possible. The problems with violence are that you alienate those people who are 'sat on the fence' about what you are demonstrating for and you give the politicians the easiest of excuses for whatever bully boy behaviour they wish to deploy. The media will rubbish you, the government will rubbish you, other people will rubbish you and you'll end up with zero support and a load of broken bones for your trouble.
A successful mass movement has to take people with it. Many non-participants are skeptical because they see a movement without any aims beyond slogans - if you throw in violent action then you look physically and intellectually bankrupt. Non-violent resistance in the face of police brutality is a brave move for anyone and i understand why not all people are capable of, or want to take this option, but what it does do is to lay bare the mechanations of the state and its enforcers for everyone to see. Those capable of it should be applauded, not denigrated for an apparent lack of courage.
The lack of violence #occupy has had so far has left the police, local mayors and politicians looking like they are the thugs with no response other than to beat on passive disobedient civilians. The way that has played out in the media is massively important for the movement, the lack of violence has allowed it to gain momentum and get others involved where scenes of it would have perhaps put them off.
I know there are many sound intellectual reasons for using violence again the state, but people should be copying what is working and that means not using violence.