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".