I feel like I need to reply to this thread, because it's a question that I've had to deal with lately. I think when people say, "Why boycott a morally bankrupt (at least in practice) corporation or practice?" what they really mean is, 'Why deny yourself something when no one but you will ever care that you're doing it?' Basically that argument relies on the idea that if it doesn't effect anyone besides yourself, it's not worth doing. That's... an argument, but when I'm boycotting something I'm not just doing so because I want the companies or individuals to cease engaging in the activity I find so repugnant-- though that's certainly a hope and a large part of it. I'm also doing it for me. Just because this kind (the at issue thing) of activity is permissible in society, doesn't make it well-considered or moral or... anything, really, beyond accepted. Why should I participate in something that is clearly ill-conceived and innately deleterious? Why should I- this organism- do this when it's repugnant to me, when the effects it has on the human race, the world, or my individual self are obviously harmful? The fact is, I'm not a big fan of most people-- largely because culture inculcates individuals into a safety mode where humans don't have to deal with questions of morality or existence or challenging the dominant paradigm. This tends to create a herd mentality. A mentality where people say, "Why are you following your beliefs? It's not like the ethically bankrupt companies/practices care, and you're just making those of of who choose to remain ignorant uncomfortable. Not that there's anything objectively wrong with ignorance. If you can manage not to hate yourself, then you have the right to your comfort. It's just... something I'm scared of ever wanting myself. And all that said, sometimes you still have to compromise. Anyway, to answer your question, I think your question boils down to perspective... like most things. If you choose to see your actions as futile if there's no external consequence, then I guess individuals boycotting is pointless. But if you're looking for internal validation that comes from living in accordance with your beliefs and self, then obviously it's far from pointless.
One person boycotting absolutely makes a difference. Believe me. The bottom line is watched closely by business. Example, not buying a product or service from one company causes you to buy from another company. Market share has changed And with the internet its so easy to share with friends each others actions and ideas. Your economic and political choices make an impact. Dont let anyone tell you it doesn't.
I like Lenin not for what all he did, but for the fact that he was a damn genious. One man isn't worth anything, but he knew that if he had amassed an army of pissed off workers, he could have the Czars balls in his grip before the morning came. In conclusion, no, One person is useless. In a populous of 300,000,000 (assuming you are American.), your boycott doesn't make much of a difference. Not trying to be offensive or anything...
But if 300,000 even stop buying crap it can make a difference. Wall St. would like you think it doesn't matter, but it does. They are seeking a market if you seek to not buy their crap they lose their power.