You really don't know of any?Which peaceful demonstrations have been successful?
I've seen many such demonstrations during the Viet Nam war.
I say they worked.
Would burning your car or breaking your windows have been better?
Of course riots happen.There are many movements where the "peaceful" side of the movement worked within a context of violence: MLK had Malcolm X, Gandhi had Chandra Bos, etc.
Or Boston.
The effects of riots vary. And riots tend to happen when other avenues of protest aren't available, or have proven themselves ineffective.
I say they're best avoided.