Or there are the people who won't read textbooks, or Wikipedia, or whatever, because they feel like everything ought to be covered in class well enough that they can just show up every day and get a good grade. I can not think of any good pedagogical reason why this should be so, and indeed, it usually isn't.

I've often found that this is so. I do try to read my textbooks, at least the assigned readings, because...well, because you're supposed to, I guess. But for most of my first year classes (three anatomy courses, psych 101, microbiology) just going to class was enough. (I did of course take detailed notes, with colourful diagrams, and then study from my notes afterwards. I have now bequeathed my anatomy notes to a friend a couple of grades younger.) One possible reason why this is true for me is that I like biology-related subjects, and I've always read anything I could get my hands on, and so I arrived in university to find that I already knew at least 50% of the material.

Areas where this isn't true: English classes, history classes, etc, where there are a lot of required readings that cover material not covered in class, and where there are essays or papers to be written on material that isn't covered in class. And of course there's no rule that you can get good grades without reading textbooks. It just happens to be true sometimes, for some people.