[WS:] I think this is a gross overstatement, to say the least. The "black block" tactic may provoke police moving against other protesters, but that is still a tiny minority and certainly not the "broader institutions of the left."
As I see it, the recriminations against anarchists and fellow travelers are grounded in a misunderstanding of the role of such groups in a broader political struggle. Their role is not to BE that broader political struggle bur rather to PREPARE for it by socializing people into a value system (egalitarianism, direct democracy, etc.) that is conducive to that struggle, raising visibility of issues and grievances, identifying specific targets to redress these grievances, and so on. Their relationship to the "broader institutions of the left" is akin to that between boy scouts and the army. It would be ridiculous to argue that boy scout exercises "sabotage" the army because they do not effectively engage the enemy forces. The role of the scouts is not to engage the enemy forces but to prepare young people to serve in the army that does by socializing them into a set of values (discipline, patriotism, etc.) and developing certain skills (e.g. survivalism, team work, etc.) that are useful in the army. Likewise, participation in the anarchist and direct action groups develops a set of values and skills that cane be useful in the "broader institutions of the left."
Wojtek