When was the last time it worked?

Protests throughout history have created transformational change. Women’s suffrage and civil rights in the U.S., Indian independence, the color revolutions in Eastern Europe, and the Arab Spring all depended on the traditionally powerless coming together against the powerful. The tea party started as a protest against the bailouts and other relief measures designed to rescue the country from the Great Recession. The 1963 March on Washington featuring the Rev. Martin Luther King Jr.'s "I Have A Dream" speech. The protests regarding the Vietnam war didn't end it but did showcase the growing dissatisfaction with policy.

It's not so much whether protests are effective or not - indeed, a recent Harvard study indicated they are - it's how the protests are planned and carried out that seem to be the best indicators of success. It's best to think of protests as affecting trends in thinking n policy rather than flashpoints of change.