<cough> No, the /code/ you offered was looking for a list with less than a given length with the understanding that nils might be in the list. The test for a list without nils is:
(all idfn lst)
Is it a kluge to state the test positively? Actually, negative tests are understood to be harder on the cognitive system to get right.
Meanwhile, are you sure (member nil lst) is a kluge? It sure looks readable, and it sure does work. What exactly bothers you about that?
Meanwhile, a use case means your are not just running code samples you typed in at the repl, it means you are trying to solve a problem using a computer and something came up. So here we are after iterating over the members of the library and we built up a list of overdue books per member -- no, that would not have nils, that would have an assoc entry (bob . nil). Hmmm... can you help me here, I cannot think of one.
You have not responded to the very similar example of tables. Correct, one cannot distinguish the absence of a key from a key added with the value nil. A use case is not a couple of lines of code proving that, a use case is a lookup on drivers license number during a traffic stop and...?
My point you may have missed is that without a use case not perfectly well expressed with either of the two solutions I offered above, the fundamental weakness is in offering a theoretical functional gap in a language as if it were a gap in a language.
Recall that pg is already on record as wanting things that are problems in practice, that this is a hacker's language, not a strait jacket language like C++ or Java with all sorts of theoretical angst over untyped variables.
The problem, btw, of putting an air bag in the trunk of a car is the added weight. :)