My take on some historical religious/social/political movements:

• Jesus taught a radical and highly impractical doctrine of love and disregard for one's own welfare. Paul took control of much of the church that Jesus' charisma had built, and reworked this into something that could function in a real community, re-emphasizing the social mores and connections that Jesus had spent so much effort denigrating, and converting Jesus' emphasis on radical social action into an emphasis on theology and salvation.
• Marx taught a radical and highly impractical theory of how workers could take over the means of production and create a state-free Utopia. Lenin and Stalin took control of the organizations built around those theories, and reworked them into a strong, centrally-controlled state.
• Che Guevara (I'm ignorant here and relying on Wikipedia; forgive me) joined Castro's rebel group early on, rose to the position of second in command, was largely responsible for the military success of the revolution, and had great motivating influence due to his charisma and his unyielding, idealistic, impractical ideas. It turned out his idealism prevented him from effectively running government institutions, so he had to go looking for other revolutions to fight in while Castro ran Cuba.

The best strategy for complex social movements is not honest rationality, because rational, practical approaches don't generate enthusiasm. A radical social movement needs one charismatic radical who enunciates appealing, impractical ideas, and another figure who can appropriate all of the energy and devotion generated by the first figure's idealism, yet not be held to their impractical ideals. It's a two-step process that is almost necessary, to protect the pretty ideals that generate popular enthusiasm from the grit and grease of institution and government. Someone needs to do a bait-and-switch. Either the original vision must be appropriated and bent to a different purpose by someone practical, or the original visionary must be dishonest or self-deceiving.

There are exceptions to this pattern that, I think, prove the rule when you look at them more closely:

• Hitler was simultaneously an idealist and a highly-pragmatic Machiavellian. His plan, spelled out in Mein Kampf, was to use hatred of the Jews to tap the energy of the German people, implement the reforms he desired, and stamp out the Jews before people had time to start feeling sorry for them and questioning themselves. It is a little more complicated since he really did hate Jews (I think), and the reforms he desired were related to his views on Jewish vs. German nature. I don't think it's a good example, since Hitler might well have had to have been replaced for the Nazi state to have held onto power if it had won the war.
• Mormonism and Scientology were each also founded largely by a single person who had, let us say, an idealistic exterior and a pragmatic, manipulative interior, combining the two roles in one person.
• MIRI's conception of Friendly AI is benevolent and idealistic, but implementing its program would require a worldwide permanent police state (in order to prevent anyone else anywhere in the world from ever constructing an AI). The free access we have now to computing power and scientific literature would need to be forbidden.

And then there are just exceptions:

• The American Revolution had a charismatic military figure in George Washington, but he wasn't very political. He was idealistic, but not impractical or messianic. AFAIK, the Revolution didn't promulgate any ideals that it failed to deliver on.
• I'm unclear on whether the French Revolution and Rousseau / Robespierre fit this pattern. I think we would need to distinguish between conscious and effective idealism: Robespierre believed in high ideals, which he used to rationalize his pragmatic, Stalinist actions. Also, Rousseau was pragmatically-minded, but had no practical experience with power.
• Anti-slavery, abolitionism, women's suffrage: Their objectives were so simple and clear that there was no way to obliterate the original message.
• Objectivism and LessWrong: Similar cases which need more analysis of what motivate their members. There's something meta here; when rationality itself is the totem, perhaps rational programs can tap idealistic energy directly. But there is an undercurrent in both movements of "Enderism" (from Ender's Game), a combination of resentment and conviction of one's own vast intellectual superiority, which may function as a secret shared "idealism".

One interesting aspect of the pattern is its hysteresis. Once idealism has been successfully co-opted, the resulting organization can continue to siphon that credibility indefinitely, while dismissing its more radical demands.