"When I grow up ... I'm not going to vote. That way I can complain when the government doesn't represent me. Then, when everything goes down the tubes, I can say the system doesn't work and justify my further lack of participation."
-Calvin & Hobbes
Purity is easy, so long as you do not act in any significant way. Refusing to commit oneself to significant political action is also easy, so long as you justify it with claims of total purity. To commit oneself in a presidential election while remaining ideologically "pure" is essentially impossible. Neither candidate has ever resembled anything close to a "pure" progressive or conservative in the history of this nation. The usual means of self-righteously advertising one's purity is the protest third party vote (which has almost zero impact) or providing simply a vote and -nothing- more (which has marginally greater impact). Rarely will you find a self-identifying "purist" who volunteers for or donates money to a major party presidential nominee. More often you will hear those "purists" issue loud proclamations about their principles being violated, and the great risk posed thereby to their nose cartilage as they are forced to vote on election day.
Today the case in point is the Powell endorsement. People will take great delight in pointing out their progressive bona fides whilst crapping all over the endorsement, since it's perfectly reasonable to despise Powell if you are a progressive: he behaved despicably on every controversial issue with which he was directly concerned--from My Lai and Dick Cheney's loopy Gulf War I "strategies" to his stint as a prop comic toting vials and cartoon trucks at the UN.
However, on DU especially, purists often move beyond denunciations of mostly-bad individuals like Powell to denunciations of those who are marginally bad at worst. John Conyers is your sworn enemy these days, if you're pure; he must be mocked at every step by reference to "strongly worded letters." JFK Jr. is an evil corporate stooge if you supported Obama in the primaries, given his Hillary endorsement. Bernie Sanders and Al Gore are spineless establishment sell-outs, if you believed immediate impeachment was the proper road for Congress to take this term. Even Dennis Kucinich must be written off by the obsessively pure--his evolving position on choice and his flag amendment vote cast him directly as an enemy of the people in the eyes of some.
The beauty of purity is that you can write off anyone and everyone who acts politically on a national level. There is no national politician in any of the three branches of government who would pass the purity litmus test. Purity is a boon to activism, as it allows one to make the strongest case in marketing terms for one's beliefs. You won't see the most prominent activists describing the opposing argument completely, accurately or fairly because a primary tool of activism is to dishonestly market a pure stance. There's nothing wrong with that--it pushes those with actual political power to lean their moderate policies in the desired direction. The other great advantage possessed by activists is that they rarely hold actual political power, and thus they are not constrained by the necessities of making policy--compromise and practicality.
Too often, DUers post on this board as activists, and I'd argue that such isn't necessary or helpful. We shouldn't confuse the debate in a discussion board run by and for progressives with the unfriendly and moderate national debate. We shouldn't need to obfuscate, falsify or purposely misinterpret opposing arguments. We shouldn't need to resort to personal attacks and righteous invective with one another. When writing, calling or protesting our elected leaders, all such tools are appropriate. But when discussing important issues amongst one another these tools of activism corrode honest debate and turn what should be enlightening discourse into a petty war of marketing and talking points. We see it all the time here. Take two posters who are equally "pure" ideologically. Say one examines the practical pros and cons of a bill whilst the other righteously rejects the entire framework of the bill as establishment cowardice. What will one learn from the other? Nothing, and two posters who agree in essence will disagree superficially, usually for many posts and perhaps resulting in a mutual ignore. What good does this do?
In other words, nobody's ego should be at stake here. The necessary tactics that go with a pretense of total purity are brilliant for activism but anathema to worthwhile discussion of what actually should be done to advance our interests. A line has to be drawn, of course, to define which means are acceptable for our ends, and the "pure" play a supremely important role in questioning those assumptions. However, we'd be better of if we could discuss matters openly, without fear of direct personal attacks on our political identities.