It gets media attention to the boycott itself, perhaps, and to the vague idea that "people disagree with John Mackey", but it doesn't contribute to the discourse. It's the equivalent of John Mackey making a poignant and eloquent speech, and a mob of campus radicals shouting him down. One of these sides is trying to actually say something, and the other side is just screaming "shut up" at the top of their lungs.
Refusing to buy products from John Mackey, or from Ben & Jerry, or from the Dixie Chicks because you disagree with their politics is the height of arrogance: "I am so 100% certain that the federal government should be running a health insurance company (or blowing up Iraq, or legalizing oil-drilling in Alaska) that I want you to lose your livelihood for ever expressing an opinion to the contrary".
> I am so 100% certain that the federal government should be running a health insurance company (or blowing up Iraq, or legalizing oil-drilling in Alaska) that I want you to lose your livelihood for ever expressing an opinion to the contrary
That is not at all what is says. What it really says is this
> I am so 100% certain that the federal government should be running a health insurance company (or blowing up Iraq, or legalizing oil-drilling in Alaska) that I will not voluntarily support you with my hard earned cash for expressing an opinion to the contrary
Yes it is. It gets media attention to the opposing idea.
> It crosses the line into incivility.
No, it does not. Refusing to buy products from people you disagree with, and convincing others to do so as well is perfectly civil.