U.S. form of government has a built-in anti-revolution factor with the power (yes, Andrea I know you don't agree with this part) of the Supreme Court able to over turn laws, and side with common people against large corporations.
I agree that practical application of a political philosophy is often unworkable. Your quotation of the communist manifesto is a great example of one of those systems.
As for 'way you would want to resort to that with a democracy when what you what you could be replacing it with could be far worse' Well people who want to seize power are never the ones out in front of a battle or protest. They let the true believers take the risks and then if they are successful with the over throw of those in power. They come from the back, and take over. People don't have a crystal ball to see what they are replacing a government with is far worse. As to why in the first place, people in power are usually greedy for more power, those with no power just want some power period.