Many US libraries have audio (CDs, cassetes). Most of them are just as senseless.
I once checked out le Carré's Tinker, Tailor, Soldier, Spy. It was something of an action novel, and it was entertaining. Was that one OK?
Given that the private sector does seem fully capable of providing libraries to those who want them, why do we subsidize them?
The question is rarely as simple as "could the private sector do it?"
Healthcare provision is rivalrous and excludible, ask any poor American, doesn't mean it shouldn't be universally provided by the government.
Education is rivalrous and excludible, look at private schools - doesn't mean it's wrong for there to be state schools and laws making children attend them.
The only thing that matters is whether the public benefit outweighs the burden on taxpayers.
(I'd be fine with no government funding for libraries in general; it seems like something which should be privately or charity funded to keep them safe from censorship and political control.)
You're being facetious, but this is precisely the question libraries have always faced, through thin times and fat, regardless of the social and political winds. They have only so much shelf space and face the simple fact nobody will come to a location that doesn't offer them anything they want.
There's a very old political debate at bottom: Should a government institution give the taxpayers what they want, or should it focus more on giving them what the better-educated types decide they need? Go the first way and you're competing with Wal-Mart and Barnes & Noble; go the second and you have a wonderful temple to Sanctioned Knowledge that, to a first approximation, nobody really cares about.
So, yes, what are 'sensible' holdings? Do you have any answers?
My hackles rise whenever I hear someone tell me what's worthwhile and what's not. But I do recognize practical necessity. I don't believe there's anything magical about the book/text format over electronic/other formats, other than "that's how it's been" up until recently.
I would be pleased to see "Fart Proudly" in any library's holdings, regardless of format.
Don't forget that libraries host community events, children's books, story time, literacy programs, and give poor people access to computers, printers, and the internet.