*nods* Yeah, I see that, in terms of sensitive items (and of course all it takes is a shifting of political winds to change what's sensitive). It's a bigger threat with electronic records, I would expect, because it's easier to do stuff en masse, but of course if the set of books you want to snoop on people for is small, the analog version was plenty risky.
The thing I liked about these cards was that they made tangible something about the community function of a library, showing you how you and other people are related. It wouldn't surprise me if it's a general principle that the practices which can generate this kind of feeling also present risks.