I don't understand what part of American history is worthy of being idealized...
Innit a lot of it not so much idealization as being invented whole cloth, though? Actual
myth instead of nostalgia or idealized history. The one that crosses my mind most often is the "christian founders" silliness or certain groups raging hardon for the puritans in defiance of actual history, but that's far from the only ones...
Re: MSH: Better put, in a lot of ways. My issue is more with the stuff that's just blatantly wrong and being pulled out of someone's hat to support some kind of agenda, yah. Shit gets annoying.
Or perhaps idealization is inherently a bad thing, since it's not indicative of reality.
Honestly, it depends on what's being idealized and what you're trying to accomplish by doing so -- idealization is almost necessarily a thing done as a means of social engineering -- and it's a hell of a tightrope to walk.
We idealize rationality and generally consider that a good thing, ferex, but most of the defensibility of that idealization comes from practical considerations (getting the best results out of doing so) rather than any particularly close connection to reality. Aristotle still has one of the better one-line descriptions for humans, and "rational" isn't involved

I guess you could say that it's no more an inherently bad than what is existent in reality inherently a good thing. Our reaction on a social and personal level and the extent either methodological tool is beneficial depends entirely on the goal, and even then may shift in importance based on which step toward the goal you're currently addressing.