Michael, I mostly agree with your sentiments. I say all the time when addressing the need and the ability to eat local in the winter, “the reasons to eat local do not diminish at various times of the year”, or “why only eat local now?” Likewise, how big is your impact if your eating local lasts a week or so. Also, as you and I are both wont to fret over, these challenges tend to promote an extreme form of locavorism. The, “it's only a week so I can give up chocolate,” idea. I would rather people be practically local all the time than absolutely local some of the time.
I do think, however, that there is much good to these challenges. What was the mantra I heard over and over again when I participated in David Hammond's locavore piece on WBEZ, “it tastes better.” I heard the same exact words today interviewing two chefs, “it tastes better.” To slightly paraphrase, or at least change the intent, from Upton Sinclair, aim for their heads but hit them in the stomachs. Nothing more will convince you to eat local more than to eat local.
Eating local is a challenge. It requires finding new places to shop or making time to be at a market. It requires time to shell beans or roast and peel beets–two foods in season now. We all have to start somewhere, and these challenges give people the, well, challenge to start. It also helps people adapt by seeing how others are adapting. It makes the resources and the supplies that exist more notable, prominent.
Lastly, do not dismiss the motive of a challenge. A lot of people, myself included, got their start eating locally inspired by a challenge, a dare. Could it be done. For me, it was one month, then two months, then several months, then a season, a year. We have not turned back from the initial challenge. Our notion of eating local, our purity, increases constantly. At first, our local did not include local meat. We fixed that. We have added more local grains to our repertoire. This year my wife has canned a lot of tomatoes to replace the non-local we were still using. If you cannot start with a challenge, when can you start?
So, yes, do not get caught up in the histrionics of a challenge. Do not be so holy for a week that it can over come a future of inaction. On the other hand, build from the momentum of the challenge undertaken by others. Learn together. Be inspired by what you can do in a few weeks. Then, most importantly, keep on doing it.