I have a (new to me, but old) bike that has a Pletscher mousetrap-style rear rack. Is the "trap" just for small cushy things? A hoodie, for example stays put, but I'm not sure a briefcase or a book would. Is it supposed to hook onto other components like certain styles of baskets?

What's the story behind these racks? They're obviously solid and good quality, since so many are still around. But I gather that some don't like them -- what gives?