Yeah! Let's all protest by breaking other people's stuff! Smash some windows! Torch some privately owned vehicles!! Yeah, that's showing the man!!
So the people who get gassed and beaten and shot with rubber bullets and subjected to hearing-damaging devices at these protests are all, or even mostly, black-clad ninjas who are doing the vandalizing and molotov-cocktail-throwing? Not so, and in a few cases in past protests, there has been reasonably convincing evidence of police planting agent provocateurs who then participate in the illegal activities.
The cost of security in Toronto for the G-8 and G-20 was roughly $1 billion. Windows still got smashed, cars still got burned, but not to worry I guess... the conference itself went on undisturbed.
If that's option A, I think just about any option B would be preferable, regardless of who's at fault for causing the damage or making the government think it needs that much security. It's not like these sorts of protests are unforeseen. How about teleconferencing?