As an example, the San Francisco Zen Center's farm, Green Gulch, has a Sunday program that's quite nice:
http://www.sfzc.org/ggf/display.asp?catid=3,123&pageid=239
Gentle instruction, a short lecture, and then the option of casual tea and conversation.
If people find themselves comfortable enough with a single sit that they're saying, "Gosh, I want 3 days of this," then more power to 'em. But that definitely wasn't my first experience. I needed a more gradual path.
My concern about the harm of an unprepared person taking a 10-day class is different. Most of the people I know who went from doing nothing to trying a marathon are no longer runners. Either the difficulty and pain or the experience of failure were enough to dissuade them from taking up a regular running practice. If someone jumps into a 10-day retreat with no prior experience, I would be concerned about something similar happening to them.