We touched upon this in another thread recently, but it was kind of off-topic there (the one on Anarchism). So I wonder we might continue here in this thread.
I work with Judy Rebick, so I generally read her blog posts every day, and when she posted today's, I thought, hey, I'll bet babblers will have something to say about this.
I remember back in 2001 we had a similar discussion (wish I could access it so that people could read it for background, sigh). People talked about "diversity of tactics" and "protesting zones" etc., where the militants would be up front, and people who didn't want to throw rocks or smash windows or whatever would stay in designated zones. A pretty big debate followed, between those who were okay with smashing windows or rock-throwing (or at least didn't want to condemn the practice), and those who felt it was bad optics and bad for the movement.
I feel like it's not really much of an issue these days - I've been to a good number of protests in Toronto in the past few years but none of them have been violent, so my feeling is that the "black bloc" thing is kind of passe. Which I'm happy about, because I like to take my son to the occasional protest, but would feel pretty uncomfortable doing so if I thought there was a chance of rock-throwing or tear gas.
Anyhow...here's Judy's take on the subject, but of course other views are welcome too. I would also note that it's important to take into account the possibility of agents provocateur, such as that rock-thrower that was unmasked at a protest a few years ago, who turned out to be an undercover police officer.