While discussing the idea of sheltering kids on the Nova Unschoolers email list, someone posted a link to this great article by Alfie Kohn. He talks about how the "Better Get Used To It" principle is used to justify the continuation of bad educational practices and that it immediately shuts down all arguments.
Well, he makes the argument much better then I can, so definitely check it out. And while you are at it, be sure to check out the rest of his site. Great stuff. Really great stuff.