I agree that many people who brand themselves as "locavores" are people that are driven by pure sentimentality, for a nostalgia for a past that never was, but there are other people that care about what happens to their food in transit and the steps that have been taken to produce food that will survive said transit, whether it be by breeding, processing or whatever else.
One of my favorite things about living outside the US is that towns and cities in the places I have lived were engineered for human habitation in the absence of cars. Simply put, most American towns and cities make no sense at all without transportation. Currently I live in New England, which is of course one of the oldest populated areas of the US and even here it's a pain to get out to the store to get groceries or whatever. If you scan the landscape though, it's pretty clear that most of the surrounding area used to be farms and the murals in town show the old wet market as it used to exist. Now, subdivisions bear the names of the family farms that they were built on and the lone market in town survives by catering to college students and the faculty of the university, by way of organic foods and gourmet food products. That's all well and good if you have money, but for people who can't justify the cost of shopping there, the closest market is about 10 miles away in another town.
For me, locally sourced food is not at all about the carbon footprint, it's about keeping and creating jobs in areas that historically produced food and most importantly of all to me, freshness and variety. I used to go to the markets in Vietnam as early as I could to get the freshest food possible. This meant that I'd see farmers bringing in vegetables with the dirt still on it and be able to pick the best chickens before they were slaughtered. Hell, I was even able to get a piglet at a market. I didn't eat him though, I'd just always wanted a pig.
And the fish. There are few things in life as good as freshly caught fish, and the ability to walk up to the fishermen as they pulled into the harbor with their catch is something I'm not sure I'll have the pleasure to do again. In New England, the catch is all flash frozen on the boat, which does a good job of preserving it, but it's just not the same.
People forget that there's people working to produce the food we eat. In a real market, relationships are formed between these people and the consumers. The customers go to the vendors and producers they trust, which in turn creates an obligation for the vendor or producer to take care of the customer in the interest of continuing to do business with them. And who doesn't like to hear that their work has brought a good experience directly from the source? Feedback is a powerful thing on a person-to-person basis, but somehow it loses power when it's filtered through an 800 number to some operator in who-knows-where.