If you're wondering why the so-called "public option" in proposed health reform plans is such a line-in-the-sand non-negotiable requirement for so many progressives, you should definitely read Mark Schmitt's brief item for The American Prospect explaining the history of this concept.

It all started, Schmitt explains, among single-payer advocates who had been convinced by pollster Celinda Lake that their approach just didn't have enough popular support to carry the day:

One key player was Roger Hickey of the Campaign for America's Future. Hickey took UC Berkley health care expert Jacob Hacker's idea for "a new public insurance pool modeled after Medicare" and went around to the community of single-payer advocates, making the case that this limited "public option" was the best they could hope for. Ideally, it would someday magically turn into single-payer. And then Hickey went to all the presidential candidates, acknowledging that politically, they couldn't support single-payer, but that the "public option" would attract a real progressive constituency.

John Edwards signed on, and then Obama and Clinton, and a hybrid proposal that added the public option to the prevailing concept of a competitive system of private insurance plans became the standard Democratic approach.
But baked into the cake was a subtle but important difference in perspective between single-payer fans who viewed the public option as the sine qua non, and other progressives who viewed it as just one of many moving parts in a comprehensive system.

That division continues today. Here's how Schmitt describes the passion of single-payer advocates for a "robust" public option, as many House Democrats call their demand:

So now this energetic, well-funded group of progressives is fired up to defend something fairly complex and not necessarily essential to health reform. (Or, put another way, there are plenty of bad versions of a public plan.) The symbolic intensity is hard for others to understand. But the intensity is understandable if you recognize that this is what they gave up single-payer for, so they want to win at least that much.

So hard-line defense of the public option is about substance, but it's also about emotion, and about people who think they've already compromised enough by accepting a system built around private health insurance. This is all worth remembering when the final deals get cut in Congress on health reform.