I agree with you that it’s not necessary for politics to be a breeding ground for disingenuousness. However, I do think that politics, by its nature, is much more likely to make people resort to it. Take a hypothetical instance of a perfectly decent up and coming politician, who is sincere in all his intentions. First comes the practicality of merely achieving power. Unless he is lucky enough to have the right connections, he must either (a) work extremely hard, or (b) work pretty hard, but tell a few small lies along the way. By “lie” here I mean the occasional flip-flopping regarding positions on various issues, to appease the more important party members.

Secondly, once he is in power, he must attempt the equally difficult task of living up to expectations. Usually when a perfectly sincere politician makes promises, they are sincere, optimistic, and not based in reality at all. Obama intends to invest heavily in finding new energy sources if he wins the presidency. But on what real evidence can he make a realistic timetable for such a plan? Imagine he becomes president for 2 terms. 7 years down the line, if he still hasn’t changed America’s reliance on foreign oil, he will be widely criticised, and forced, probably, to resort to desperate measures to distract the populace.

So I agree that disingenuousness is not necessary, but I also think it may be difficult to avoid once the everyday realities set in.

“…but is it not really cowardice? To leave politics, the arena in which you hoped to change the world for the better, over a disagreement, however large? …The opposite is much more common of course, and is certainly cowardly—pragmatic, but cowardly.”

Two very good observations. But they raise a question that you have not addressed in this post. Is it necessary for the political environment to be a breeding ground for disingenuousness?
I hold that it is not necessary. It is so today only because politics has overstepped its limits, and government has intervened in matters that are none of its business. There is no right way for a government to do what it should not have been doing in the first place (actions such as spreading democracy in the world or making education or health care or home ownership affordable). The role of the government is solely to protect the rights of individuals to act on their own judgement. When the government does more, it necessarily violates some individuals’ rights and has to start lying to justify the violations.