Coincidentally, I recently read a similar contention in a column by Thomas Friedman. Friedman's point was about environmental impacts in general, another subject dear to my heart. He talked about one person buying a Prius, versus a some people high up who got in meetings with New York's taxi organizations to start converting the taxi fleet to hybrids (mostly not Priuses -- SUVs -- which is a bit unfortunate but perhaps more practical from a taxi standpoint).
I certainly agree with the point that change is almost always going to happen faster and be more effective when it's promoted at a high level, and that for things that are ingrained habits for most people, and seem kind of minor (like what they eat and what kind of bags they get at the supermarket), it's very hard to effect massive change on a grass-roots level only.
However, there are two counter-contentions that I would make about why it's essential for all of us to act on our personal ethical beliefs.
The first one could, with rhetorical flourish, be called the Argument From Hypocrisy. With less flourish, we could call the the Argument From Moral Consistency. If I think everyone should eat only animals who are raised humanely and sustainably (and vegetables raised sustainably), then it's ridiculous if I go around telling other people I think they should do it, or setting up laws to promote it, while not doing it myself, even if I say I'm going to start doing it as soon as there are laws or other people are doing it. I don't think this argument needs to be elaborated much. To have any moral standing, I have to be acting out my own ethical standards to at least a high percentage. If I'm preaching and not practicing, what I say rightly has very little weight.
The second one I would call the Argument From Demonstrated Opportunity. If no one is trying to eat sustainably raised animals or produce, then going to Cargill or ADM or whoever and saying "Geez, we think you should raise your chickens/corn/etc sustainably", they'll laugh and say "But look, everyone is buying our corn. No one cares about that, they're happy with what they have." The fact that not everyone is buying their corn or chickens lends weight to the concept that they should change their practices. The same would be true if you go to Congress and say "You should make a law about this". If no one is doing it, they'll say "But no one does this. No one cares." Whereas, if you go to Congress in our world today, you can point to the many hundreds, perhaps thousands, of people who are vegetarian or eat only sustainably raised meat, who buy organic produce, who go to farmer's markets. You can say "You should encourage this." It also kills the argument "It's impossible to do this and make money" or just "It's impossible to do this" period.
The same is true for the New York hybrid taxis. What would have happened if no one had bought a Prius, as Friedman is basically recommending (because it doesn't make a difference, right)? That's easy. Hybrids would have been declared a business failure, something no one wanted, and there would never have been an opportunity to even discuss using hybrids in the New York taxi fleet. Therefore, those who decided to buy Priuses to satisfy their personal desires actually did make a difference: they made it possible to discuss implemented hybrids on a wide scale.
And it applies to almost any issue: if no one rode bikes on the roads, we would have nobody to point to when we ask for more bike accommodations (indeed, it would seem clear that we didn't need more). If no one doesn't have a car, if no one actually uses public transit to get around, then we have no one to point to when we ask for better transit. In fact, one of the hardest things to make happen in the transit world is a new route or a new allowance (like bikes on Caltrain). Because it's currently nonexistent or prohibited, there's no direct evidence that it's needed, and so usually (since these things cost money) it's assumed that it isn't needed. There is latent demand for such things, but latent demand has to get very strong and very vocal to be as powerful as demonstrated demand that we create by our own actions. Latent demand is calculated every time a new product is launched or a company is started -- but trying to find latent demand is risky. It's much easier if there's a clear need for a product or service, and we create that proven need for services we desire if we act on our beliefs.
Change has to come from the ground up, as well as the top down. In fact, I would bet that almost all top-down change ultimately originates at the ground level. That's why we write letters to (and lobby) our Congressional representatives. That's why Leo made all the West Wing staffers listen to the fringe people for one day a year. We all have an obligation to do what we can, and tell others what we believe, or most top-down change will never even get started.