Since people are ruling over other people these people aren´t satisfied how a king, politician or company boss rules them. And since mankind isn´t always rational and compromise willing people have to give their opportunism a bit more expression, they have to protest. So how you see protest in general is good, especially in modern democracies where everybody has a voice. Protest is also often the only possibility for many people to be heard by the powerful, but way and manner of the protest strongly matters whether the protest is successful or not.
Placid protests like demonstrations or protest parades are always legitimate to express an opinion. This protest is especially beneficial in case of making politicians attentively about an opinion or problem. But in chase of known positions, problems and hardened fronts any form of pressure is needed to move someone position.
This pressure can range from work stoppages up to a violent revolution. The legitimisation of these measures isn’t depending on how violent they are, but how high the amounts of negative influence uninvolved have to take is. In case a large part of the population of any country is not satisfied with the government and the government but don´t respond to the protesters requirements, even not after long protest and negotiations, the protester are allowed to assault governmental buildings to dismiss the government if this measures stands in context with the problems and disagreements with the government; this would be a revolution and totally legitimate. But in case the protesters e.g. burn cars of uninvolved people, as part as their pressure and protest, it would not be legitimate and successful, because the government maybe just would try to stop the protest with more violent and would not respond.
So from this perspective even a pie-in-the-face protest is legitimate, if it hits the right one. The question is how effective this or other kinds of protest are for certain objectives.
So for me protest is always something good, because people participate through protest in political processes and often it’s the only way for them to participate. What anybody thinking about to protests has to mind are two things: First: How effective is my kind of process in order to reach my objective? and Second: In which amount are uninvolved effected? Anybody who can answers these questions with a calm concession is ready for his or her kind of protest.