I've done this arguement to death over the years, mostly coming at it from your point of view (for protest in this country at least), and am needing to go to bed now, so all I'll say is that I hope you're right, but fear you're wrong.
this government has shown that it takes no notice of whatever scale of peaceful protest you can manage, or at best pays lip service to doing something to acknowledge it while actually doing the opposite. At the same time they're intent on removing / have already removed a huge amount of our old freedoms to protest and object legally and peacefully to their policies (and effectively). When these policies include starting illegal wars that lead to the deaths of in the region of a million civilians, is doing sweet fuck all about climate change that's going to kill many more, and instead of moving towards sustainable development as agreed at Rio, have persued headlong neoliberal policies that have led to the virtual collapse of the global economy and yet still they persist in mortgaging this and the next generation upto the hilt in some futile attempt to bail the failed system out... at some point, maybe, non violence stops being the logical tactic*.
Also kinda depends what you mean by non-violence, as you seem to be conflating targeted destruction of property with actual violence.
*not saying we're at that point now mind
very quickly, since I too should be asleep right now,
1) the non-violence works, for example, the Bi'lin protests in Palestine-Israel held jointly by Israeli-Palestinian Jews and Arabs. search the Middle East forum on my posts there on the non-violence movement.
2) Best reason to do it - only the authorities are the ones meting out the violence. Agent provocateurs and random violence starters can be rooted out easier this way. We live in Britain, not Israel-Palestine. We need to build public support, at this early stage, all is not lost, but it could be lost in future if violent acts against persons or property continue Non-violent demos will encourage others who might not join in to join in. If people think they're going to get hurt, either by acts of other protestors or by police, they won't join us and we lose.
3) Don't confuse non-violence with a civil disobedience, e.g. squatting, sit-downs, repeated marches/demos to keep issue in news. Throwing missiles which might injure someone on the demo or even the police is potentially violent.
4) Fighting-fund - money is needed to fight ineveitable court cases - see Anarchists against the Wall website for news of how long a fight needs to be kept running. It took 2 years to get the Bi'lin demo - held every week by - before Channel 4 news did a long piece on the issues.
By non-violence, I mean no throwing any missiles, smoke bombs. No point really in destroying property - private or public, but fine to dismantle illegal walls/fences (see Bi'ilin protests - where farmers cut off from their lands and denied access via checkpoints despite court orders saying farmers must be allowed access)
By civil disobedience I mean disregarding demands by authorities and demonstrating regardless, keeping spirits high, and getting articulate messages across via all media at disposal.
All of a sudden, riot police move in. Why? 
