While the person who actually asked it was being facetious, not actually wanting an answer and thinking they had stumbled onto some sort of incredibly witty rejoinder, I want to actually answer that question.
For one, as this great Al Jazeera piece from last year points out, riots have actually been extremely effective in creating social change throughout American history. Or as many pointed out in the wake of the murder of Michael Brown, the world probably would have known nothing about Brown's death had the community not poured into the streets demanding justice. One could pretty easily make the same argument that Gray would just be another statistic in our ever-expanding list of citizens killed at the hands of police were it not for the good people of Baltimore loudly proclaiming they've finally had enough.
But what the titular question of this post most often refers to (at least in my experience) is that the employment of violence and property damage will result in bad optics; that is, the great white American public hive mind will be turned off from what is otherwise a legitimate cause because of the actions of a few (if you've read anything about Grey that hasn't invoked this argument, you're doing a very good job seeking out a very particular kind of media source).
Beside the obvious fact that that it's pretty fucked up for one to be more upset over a CVS being on fire than one is about a person begin murdered by the very forces that are supposed to protect them, the optics argument is at best incredibly over-optimistic, but more likely disingenuous. Because the very premise of the argument is quite easily proven false; the only way you can argue violence and property damage may damage the chance at achieving some measure of justice is if you assume white America was primed and waiting to fully tackle our nation's vast and entrenched racial injustices. But now they're changing their minds because of the unrest in Baltimore. But if not for that unrest! Oh, if only the CVS hadn't been torched, then the majority of white Americans would have been pouring into the streets, demanding change and not settling for milquetoast reforms. If only! But alas, because of some minor violence, all good people must simply turn up their noses at what's going on and ignore the underlying issue. There's no other choice!
In case my dickish sarcasm hasn't made the point clear enough, the assumption underlying the optics argument is demonstrably (and frankly, laughably) false. In fact, it's pretty easy to view the history of police violence as a natural experiment: we as a nation are certainly not short on instances of unarmed Black people murdered by police. And in the vast majority of those instances, there are no resulting riots, violence, or property damage. Yet somehow, even without these damaging optics, there haven't been widespread demands from white America for tearing down the edifice of racial injustice. Why, it's almost as if the presence of violence and property damage doesn't cause people to not call for changes but instead gives people an excuse to justify their racist views!
To steal a great quote I saw on the face books this morning (attributed to a Dr. Kevin Browne, though a quick google search tells me there's a lot of people with that name and I don't know which one it said it): "When police departments protect white property BETTER than they protect Black people, they're actually punishing us for NOT wanting to be white property. 'How dare you not want to owned by us?' 'How dare you think yourselves more valuable than things?' 'Look how well we protect our things.'"
But the bigger point of all of this is to ask where are these people calling for calm and nonviolence when the police are routinely beating and murdering innocent citizens? Why are they only concerned with calm and order the second it's Black people who are causing damage (damage which pales in comparison to the actions of the police)? As the incomparable Ta-Nehisi Coates argues:
When nonviolence is preached as an attempt to evade the repercussions of political brutality, it betrays itself. When nonviolence begins halfway through the war with the aggressor calling time out, it exposes itself as a ruse. When nonviolence is preached by the representatives of the state, while the state doles out heaps of violence to its citizens, it reveals itself to be a con. And none of this can mean that rioting or violence is "correct" or "wise," any more than a forest fire can be "correct" or "wise." Wisdom isn't the point tonight. Disrespect is. In this case, disrespect for the hollow law and failed order that so regularly disrespects the rioters themselves.So what does burning down a CVS have to do with racial injustice? Seems like quite a bit from where I'm sitting.