I think calling people, who are by far mostly peaceful protesters, "rioters", is harmful to any possible progress because to many it would invalidate the grievances of the protesters (because, unfortunately, poisoning the well is totally effective as a rhetorical device and affecting public opinion, even if it's a fallacy).
We need to make a sharp distinction between protesters and rioters. I have no problem with protesters. I am not calling peaceful protesters rioters, I am calling those who destroy businesses and property and assault business owners in the name of this cause "rioters."
I disagree that condemning the rioters invalidates the grievances of the protesters. I hope you agree that just because someone supports X, doesn't mean that they are condoned to achieve X at virtually any cost.
I'll call that collateral damage and insist that it doesn't affect the righteousness of the cause being pursued, much as, when a bomb is dropped on a strategic bridge, we don't care about the loss of life of non-combatants.
I can tell you that as someone in the Air Force, we do care about collateral damage. Even if we were targeting bin laden himself (I know he's dead) we don't have a blank cheque to, say, destroy a city in order to kill him. There's a serious discussion to be had over how much collateral damage is permissible, but no one is saying that
everything is acceptable in the name of achieving an objective or that the loss of life from collateral damage doesn't matter.
In any case in this scenario we're talking about the actions of individuals, not potentially imprecise bombs or possibly faulty intel being dropped on an enemy. Collateral damage implies a degree of inevitability, but we need to be seriously careful about this whether we're talking about an actual war or social change. Reasonable people agree with fighting Hitler, but disagree with some of the bombing runs - say, Dresden. Be careful in the name of fighting a monster that you don't become one yourself. This is always one of the dangers of war.
If things don't materially change so that US society becomes more just because the political institutions are either a) incapable or b) unwilling to affect change, then riots definitely become an option in my book and ethically defensible. Just more collateral damage.
I would strongly advise using other means to achieve your goals. I think rioting and destroying local private businesses is almost never excusable - even if the system is unchangeably rotten to the core. I wouldn't have excused Jews rioting in Nazi Germany and destroying German businesses even after the Nuremberg laws were passed. It just wouldn't have been the proper response on several fronts, and I say this as someone with family killed in the Holocaust. If we're talking about targeting government officials that's a different story.