This is an interesting psychological concept.

It makes a lot of sense. I've attempted engaging in conversation with people in the past regarding the effectiveness (and safety) of protesting.

I can think of a lot of reasons why protesting may actually be counterintuitive.

• Unpredictability of the herd mentality can become quickly dangerous for self preservation.
• Opposing forces can quickly turn to brutal lethal force and at this point odds of escaping are far less than if you were never there to begin with.
• Let's the enemy know how well their plans are working.

Those are the key factors I think of when it comes to joining in with big groups, especially one's with a political agenda. I'll make an exception for a nice concert or food event once in a while :)

That's not to say there aren't good political agendas, which try to overcome evil ones, but I've heard the pen is mightier than the sword.