The worst thing an idealist can be is practical. I see this problem nearly every day when people are trying to comply with laws, rules, regulations, standards, or what-have-you. You fail to meet the objective, so you smudge reality and make compromises. Well, we cannot reasonably drive under the speed limit so let's arbitrarily make up our own limit (10 mph over) and stick to that. That way we endure the hardship of having to comply with something but not the extreme and unattainable hardship of complying with the real thing. Nevermind that some people actually obey the speed limit; that's just anecdotal evidence or a fluke or some other excuse you can use to disregard the fact that what you are claiming is impossible is actually possible.

That's an example of a rule mandated by an external power (the oppressor you theoretically owe your allegiance to or the oppressor you are on loan to). Where idealism really comes into play is when people choose their own oppression, be that a formal religion, moral code that they got from a pamphlet somebody was handing out on the sidewalk, or other voluntarily-adopted standards of behavior.

Then you end up with raw-food vegans who eat pepperoni pizza twice a week, environmentalists who drive cars, PETA members who keep pets, feminists who are lapsitters, Christians who sin, people who claim that things are best-effort, and people who claim that things that are obviously part of other things are not really part of those things.